(CNN) -- Kofi Annan, whose initiative to forge peace in war-ravaged Syria failed to take hold, said he has resigned as the U.N. and Arab League joint special envoy because of "increasing militarization on the ground" and "the clear lack of unity" at the U.N. Security Council.
Annan said he told U.N. Secretary-General Ban Ki-moon and Arab League Secretary-General Nabil el-Araby that he didn't want to renew his mandate when it expires August 31.
Syria state-run TV said the nation "expresses regret for the resignation," and Ban said he and el-Araby are looking for a successor to take on the "crucial peacemaking effort."
In Geneva, Annan said he accepted the job despite its challenges, a task that some had dubbed "Mission Impossible." He said it was a "sacred duty" to bring about peace, end the killings and abuse of civilians, and set a course toward political change.
Syrian army soldiers take control of the village of Western Dumayna north of the rebel-held city of Qusayr on Monday, May 13. Syrian troops captured three villages in Homs province, allowing them to cut supply lines to rebels inside Qusayr town, a military officer told AFP. Tensions in Syria first flared in March 2011 during the onset of the Arab Spring, eventually escalating into a civil war that still rages. This gallery contains the most compelling images taken since the start of the conflict.
Syrian civil war in photos
"The severity of the humanitarian costs of the conflict, and the exceptional threats posed by this crisis to international peace and security, justified the attempts to secure a peaceful transition to a political settlement, however daunting the challenge. The increasing militarization on the ground and the clear lack of unity in the Security Council have fundamentally changed the circumstances for the effective exercise of my role," he said.
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"Yet the bloodshed continues, most of all because of the Syrian government's intransigence and continuing refusal to implement the six-point plan, and also because of the escalating military campaign of the opposition, all of which is compounded by the disunity of the international community. At a time when we need -- when the Syrian people desperately need action, there continues to be finger-pointing and name-calling in the Security Council."
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The nearly 17-month-long Syrian crisis started when the regime brutally cracked down on peaceful protesters in March 2011 and morphed into a nationwide uprising.
Among the five permanent members of the council, Russia and China have persistently disagreed with tough action against the regime of President Bashar al-Assad that is favored by the United States, Britain and France.
The utter failure of world diplomacy and Syrian leaders to end the fighting has resulted in a humanitarian catastrophe. Opposition groups say that more than 20,000 people have died, thousands have been imprisoned by the regime and tens of thousands have been displaced from their homes.
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The conflict is now internationally regarded as a civil war between an armed resistance movement and al-Assad's regime.
"Without serious, purposeful and united international pressure, including from the powers of the region, it is impossible for me, or anyone, to compel the Syrian government in the first place, and also the opposition, to take the steps necessary to begin a political process. You have to understand: As an envoy, I can't want peace more than the protagonists, more than the Security Council or the international community, for that matter," Annan said.
The appointment of the former secretary-general in February was supported by world powers and the Syrian government. After he took the post, he helped formulate a six-point peace plan for Syria that included a cease-fire. The United Nations established the U.N. Supervision Mission in Syria to monitor the efforts to forge the plan.
The plan called for establishing a cease-fire between al-Assad's government and the opposition, allowing humanitarian groups access to the population, releasing detainees and starting a political dialogue. It also called for government forces to withdraw from city centers.
Despite an initial and small drop in violence after his plan was announced, fighting raged nonstop. The terrain became so dangerous in Syria that U.N. monitors had to suspend their observation missions.
In an opinion piece published Thursday in the Financial Times, Annan said regional and international players have "clear common interests" in a "managed political transition."
He said al-Assad "must leave office."
Russia, China and Iran, friends of the Syrian regime, "must take concerted efforts to persuade Syria's leadership to change course and embrace a political transition, realizing the current government has lost all legitimacy," he said. The United States, Britain, France, Turkey Saudi Arabia and Qatar must press "the opposition to embrace a fully inclusive political process."
"None of this is possible, however, without genuine compromise on all sides," he wrote. "Syria can still be saved from the worst calamity. But this requires courage and leadership, most of all from the permanent members of the Security Council, including from Presidents Putin and Obama. Is ours an international community that will act in defense of the most vulnerable of our world, and make the necessary sacrifices to help? The coming weeks in Syria will tell."
The Annan initiative disappointed opposition groups.
Rafif Jouejati, English-language spokeswoman for the opposition Local Coordination Committees of Syria, said Annan's replacement "will be equally constrained by Assad's failure to adhere to any sort of plan, and the ongoing complicity of Russia, China and Iran in mass killings and exodus in Syria."
"The Annan resignation was expected, since his plans have failed. While this may cause discomfort for the U.N. and prove that it was unable to provide a solution for the Syrian people, it means nothing to the Syrian people. The revolution will continue until we achieve our goals of freedom, democracy and dignity.
"It is sad to watch the diplomatic process die in such indignity, but millions of Syrians had indicated that Assad had no intention of implementing any diplomatic or political solution to the current crisis in Syria," she said. "Annan's plan was fatally flawed for several reasons, chief among them that 300 unarmed observers under the near-constant supervision of regime monitors could not possibly have been effective in observing, enforcing or even reporting on a cease-fire, let alone the other tenets of the Annan Plan."
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Vitaly Churkin, Russian ambassador to the United Nations, said he hopes Annan uses the rest of the time on the job to be as effective as possible "under these very difficult circumstances."
"We understand that it is his decision. We regret that he chose to do so. We have supported very strongly Kofi Annan's efforts," he said. "I'm very encouraged that the secretary-general is already working on finding a successor for Kofi Annan who would be able to pick up this daunting task of putting an end to the crisis in Syria."
Ambassador Susan Rice, the United States' permanent representative to the United Nations, said Annan took on a "thankless and difficult task at great personal cost."
"When the Security Council failed to heed Mr. Annan's repeated calls for collective and significant consequences for non-compliance with its prior resolutions, those members who blocked this action effectively made Mr. Annan's mission impossible," she said in a written statement.
Catherine Ashton, the European Union's foreign policy chief, said Annan's resignation "makes clear how unfortunate it has been that the U.N. Security Council was unable to agree to a resolution."
"The EU continues to support the efforts of the U.N. and the League of Arab States and calls for the early appointment of a successor to carry on Mr. Annan's work towards a peaceful political transition in Syria," Ashton said in a written statement.
Ban, who was first to announce the move, praised Annan's "determined and courageous efforts" and said the news left him with "deep regret." He said that Annan worked within the mandate provided to him by the U.N. General Assembly and that he is "indebted to him and his team for all they have tried to achieve."
But Ban also said the government and the opposition have been determined "to rely on ever-increasing violence," and there have been "persistent divisions within the (U.N.) Security Council."
"Kofi Annan deserves our profound admiration for the selfless way in which he has put his formidable skills and prestige to this most difficult and potentially thankless of assignments," Ban said.
Annan had been secretary-general from 1996 to 2006 and was involved in other initiatives, including the mediation of the election dispute a few years ago in Kenya.
Ban said peace can come only if there's a "firm commitment to dialogue" among the players in Syria and international unity.
"Tragically, the spiral of violence in Syria is continuing. The hand extended to turn away from violence in favor of dialogue and diplomacy -- as spelled out in the six-point plan -- has not been taken, even though it still remains the best hope for the people of Syria."
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The CNN Library contributed to this report.
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Syrian troops move into Dumayna on May 13.
Rebel fighters fire at government forces in the northern Syrian city of Aleppo on Sunday, May 12.
Smoke rises from an explosion in a Syrian village near the Israeli border on Tuesday, May 7.
Multiple explosions hit a Syrian village near the Israeli border on May 6.
A photo released by the Syrian Arab News Agency shows destruction from what is said was bomb attack in the Al-Hama area of Damascus on Sunday, May 5. According to the Syrian government, Israel launched an attack on a research center in the Damascus suburbs early Sunday.
People run for cover after what activists said was shelling by forces loyal to Syria's President Bashar al-Assad in Syria's Raqqa province, on May 3.
People walk past a damaged building and multiple destroyed cars at the site of an explosion in Damascus where at least 13 were killed on April 30.
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A smoke cloud rises from shelling on the the al-Turkman mountains in Syria's Latakia province on Thursday, April 25.
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A handout photograph from Syria's national news agency SANA shows damage and debris from a mortar attack in the suburb of Jarmana near Damascus, Syria, on Wednesday, April 24. The attack killed seven and wounded more than 25, according to activists and state media. No group claimed responsibility for the mortar fire, which SANA said hit a municipality office and a school building.
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People walk past destroyed houses in the northern Syrian town of Azaz on Sunday, April 21.
Free Syrian Army fighters take positions prior to an offensive against government forces in the Khan al-Assal area, near Aleppo on Saturday, April 20.
Men inspect damage at a house destroyed in an airstrike in Aleppo on April 15.
Syrian and Kurdish rebel fighters walk in the Sheikh Maqsud district of Aleppo on April 14.
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A rebel runs to avoid sniper fire from Syrian government forces in Aleppo on Thursday, April 11.
Syrian rebels observe the movement of Syrian government forces around Al-Kendi hospital in Aleppo on Wednesday, April 10.
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A Syrian rebel runs for cover in Deir ez-Zor on April 2.
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A Free Syrian fighter mourns the death of a friend in Aleppo on March 30, in this photo taken by iReporter Lee Harper.
A Syrian opposition fighter runs for cover from Syrian army snipers in Aleppo on Wednesday, March 27.
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Syrians carry the body of a Syrian army soldier during a funeral ceremony in Idlib province on Tuesday, March 19.
Syrian rebels take position in Aleppo, the largest city in the country, on March 11.
Syrian men search for their relatives amongst the bodies of civilians executed and dumped in the Quweiq River on March 11.
A Free Syrian Army fighter looks back as smoke rises during fighting between rebel fighters and forces loyal to Syria's President Bashar al-Assad on the outskirts of Aleppo on Saturday, March 2.
Residents read Shaam News newspapers published by the Free Syrian Army in Aleppo on March 2.
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A rebel fighter throws a home-made grenade at Syrian government forces in Aleppo on February 16.
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A Syrian woman looks through a bus window in Aleppo on February 14.
Free Syrian Army fighters walk through a dust-filled stairwell in Damascus on February 7.
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A rebel fighter throws a hand grenade inside a Syrian Army base in Damascus on February 3.
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An unexploded mortar shell fired by the Syrian Army sits lodged in the ground in Damascus on January 25.
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A Syrian rebel fighter tries to locate a government jet fighter in Aleppo on January 18.
Syrian rebels launch a missile near the Abu Baker brigade in Albab on January 16.
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Syrians look for survivors amid the rubble of a building targeted by a missile in Aleppo on January 7.
A father reacts after hearing of a shelling by forces loyal to Syria's President Bashar al-Assad in Aleppo on January 3.
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Syrians mourn a fallen rebel fighter at a rebel base in the al-Fardos area of Aleppo on December 8.
Members of Liwa (Brigade) Salahadin, a Kurdish military unit fighting alongside rebel fighters, monitor the area in the besieged district of Karmel al-Jabl in Aleppo on December 6.
A member of Liwa Salahadin aims at a regime fighter in the besieged district of Karmel al-Jabl in Aleppo on December 6.
Two young boys sit underneath a washline in a refugee camp on the border between Syria and Turkey near Azaz on December 5.
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A Syrian rebel mourns the death of a comrade in Maraat al-Numan on November 20.
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A Syrian rebel takes cover during fighting against Syrian government forces in Aleppo on November 15.
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A rebel fighter fires at a Syrian government position in Aleppo on November 6.
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Rebels hold their position in the midst of a battle on November 3 in Aleppo.
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A Syrian rebel interrogates a handcuffed and blindfolded man suspected of being a pro-regime militiaman in Aleppo on October 26.
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A Syrian rebel fires at an army position in the Karm al-Jabal district of Aleppo on October 22.
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Syrian army soldiers run for cover during clashes with rebel fighters at Karam al-Jabal neighborhood of Aleppo on October 20.
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A Syrian opposition fighter stands near a post in Aleppo on October 11.
A Syrian man mourns the death of his father, who was killed during a government attack in Aleppo on October 10.
A rebel fighter is carried by his friends and laid on a gurney to be treated for gunshot wounds sustained during heavy battles with government forces in Aleppo on October 1.
Syrian rebels help a wounded comrade to an Aleppo hospital after he was injured in a Syrian army strike on September 18.
Free Syria Army fighters are reflected in a mirror they use to see a Syrian Army post only 50 meters away in Aleppo on September 16.
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A woman walks past a destroyed building in Aleppo on September 13.
Free Syrian Army fighters battle during street fighting against Syrian army soldiers in Aleppo on September 8.
A Syrian man wounded by shelling sits on a chair outside a closed shop in Aleppo on September 4.
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Syrian rebels take position near Qusayr on May 10.
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Syrian refugees walk across a field before crossing into Turkey on March 14.
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A member of the Free Syrian Army looks out over a valley in the village of Ain al-Baida on December 15, 2011.
Members of the Free Syrian Army stand in an valley near the village of Ain al-Baida, close to the Turkish border, on December 15, 2011.
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A Syrian man holds up a portrait of President Bashar al-Assad during a rally to show support for the president in Damascus on April 30, 2011.
Syrians rally to show their support for President Bashar al-Assad in Damascus on April 30, 2011.
A screen grab from YouTube shows thick smoke rising above as Syrian anti-government protesters demonstrate in Moaret Al-Noman on April 29, 2011.
A screen grab from YouTube shows Syrian anti-government protesters run for cover from tear gas fired by security forces in Damascus on April 29, 2011, during the "Day of Rage" demonstrations called by activists to put pressure on al-Assad.
Syrians wave their national flag and hold portraits of al-Assad during a rally to show their support for their leader in Damascus on March 29, 2011.
A woman sits by the hospital bed of a man allegedly injured when an armed group seized rooftops in Latakia on March 27, 2011, and opened fire at passers-by, citizens and security forces personnel according to official sources.
Syrian protesters chant slogans in support of al-Assad during a rally in Damascus on March 25, 2011.
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