(CNN) -- If North Korea continues with its controversial missile and nuclear tests, it "will move closer" to its objective of reaching the United States with nuclear weapons, according to a Pentagon report.
During recent heightened tensions on the Korean Peninsula, Pyongyang repeatedly threatened the possibility of nuclear attacks against the United States and South Korea, prompting questions on the progress of its weapons program.
North Korea's secretiveness has made it hard for Western intelligence agencies to gauge exactly what is going on inside its research facilities.
Many clues have come from the regime's large-scale tests such as the long-range rocket launch in December and the underground nuclear detonation in February.
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The Pentagon's annual report to Congress on Thursday provided an overview of the military threat posed by North Korea, but it didn't say how long it believed it would take the isolated, Stalinist state to develop a fully operational nuclear missile capable of reaching the U.S. mainland.
It described North Korea's ballistic missile program as "ambitious" and said that "the pace of its progress will depend, in part, on how many resources it can dedicate to these efforts and how often it conducts tests."
Doubts over North's capabilities
A sign of differing views on North Korea's nuclear missile capabilities among U.S. intelligence agencies emerged last month.
Addressing the House Armed Services Committee, a congressman read out an excerpt from a report by the Pentagon's intelligence arm that said it believed with "moderate confidence" that the North had nuclear weapons that could be delivered by ballistic missiles, albeit with low "reliability."
But after the disclosure of that assessment from the Defense Intelligence Agency (DIA), top U.S. officials including President Barack Obama said the U.S. government didn't think North Korea was yet able to fit a nuclear warhead on a ballistic missile.
The Pentagon report Thursday included no reference to the DIA excerpt, which didn't specify the range of the ballistic missiles that it was talking about.
The report said that the type of long-range rocket that North Korea launched in December to put a satellite in orbit "could reach parts of the United States if configured as an intercontinental ballistic missile (ICBM) capable of carrying a nuclear payload."
But it noted that "a space launch does not test a re-entry vehicle (RV), without which North Korea cannot deliver a weapon to target from an ICBM."
The December launch and the display in April 2012 of an untested but road-mobile intercontinental ballistic missile help underscore "the threat to regional stability and U.S. national security posed by North Korea," according to the report.
Like father, like son
Its authors said they didn't expect much change under North Korean leader, Kim Jong Un, from the strategy shaped by his father, Kim Jong Il, who died in December 2011.
The focus of that approach, they said, includes "coercive diplomacy to compel acceptance of its diplomatic, economic and security interests; development of strategic military capabilities to deter external attack;" and challenges to South Korea and the U.S.-South Korean alliance.
Under Kim Jong Un, North Korea has so far continued the pursuit of more advanced nuclear and missile technologies, which according to the report, the regime sees as "essential to its goals of survival, sovereignty and relevance."
The long-range rocket launch in December and underground nuclear test in February prompted international condemnation and tougher U.N. sanctions. Those measures were followed by intensified threats from Pyongyang, during which the United States said it would deploy additional missile defenses on its West Coast.
The Pentagon report also highlighted the murkiness of the North's decision-making processes -- a particular concern during periods of heightened tensions like the past few months.
"Although North Korea is unlikely to attack on a scale that it assesses would risk the survival of its government by inviting overwhelming counterattacks by the ROK or the United States, we do not know how North Korea calculates this threshold of behavior," the report said, using the abbreviated form of South Korea's official name, the Republic of Korea.
"North Korea's use of smallscale attacks and provocative acts leaves much room for miscalculation that could spiral into a larger conflict," it said.
CNN's Alison Harding contributed to this report.
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A North Korea soldier gestures to stop photographers from taking photos from a Chinese tour boat as other soldiers look on along the North Korean bank of the Yalu River near the town of Sinuiji across the Chinese city of Dandong in Liaoning province, China, on Saturday, April 6. North Korean leader Kim Jong Un has ordered the country's military to increase artillery production, a televised report out of Pyongyang showed on Saturday.
North Korean soldiers gather by the docks in Sinuiju near the Chinese border on Thursday, April 4.
North Korean soldiers patrol along the Yalu River in Sinuiju across the border from the Chinese city of Dandong on April 4.
Kim Jong Un is briefed by his generals in this undated photo. On the wall is a map titled "Plan for the strategic forces to target mainland U.S."
Kim Jong Un works during a briefing in this undated photo.
North Korean leader Kim Jong Un inspects drills by the Korean People's Army navy at an undisclosed location on North Korea's east coast on March 25 in a photo from the state-run Korean Central News Agency.
Kim makes his way to an observation post with North Korean soldiers on March 25.
Kim uses a pair of binoculars to look south from the Jangjae Islet Defense Detachment near South Korea's Taeyonphyong Island on March 7.
Kim is greeted by the family of a soldier as he inspects Jangjae Islet Defense Detachment near South Korea's Taeyonphyong Island in South Hwanghae province on Thursday, March 7, in a photo from the state-run Korean Central News Agency.
Kim is surrounded by soldiers during a visit to the Mu Islet Hero Defense Detachment near South Korea's Taeyonphyong Island on March 7. North Korea has escalated its bellicose rhetoric, threatening nuclear strikes, just before the U.N. Security Council passed tougher sanctions against the secretive nation on March 7.
Kim arrives at Jangjae Islet by boat to meet with soldiers of the Jangjae Islet Defense Detachment near Taeyonphyong Island in South Hwanghae province on March 7.
Soldiers in the North Korean army train at an undisclosed location on March 6.
North Korean leader Kim Jong Un, center, poses with chiefs of branch social security stations in this undated picture released by North Korea's official news agency on November 27.
Kim celebrates with staff from the satellite control center in Pyongyang, North Korea, during the launch of a rocket carrying a satellite, in a photo released by the official North Korean news agency on December 12.
A crowd watches as statues of the nation's founder, Kim Il Sung, and his son Kim Jong Il are unveiled during a ceremony in Pyongyang on April 13, 2012. Photos from North Korea are rare, but the country was on full display in April 2012 as it celebrated the 100th birthday of Kim Il Sung.
A North Korean soldier stands guard in front of an UNHA III rocket at the Tangachai-ri Space Center on April 8, 2012.
In April 2012, Pyongyang launched a long-range rocket, which broke apart and fell into the sea. The UNHA III rocket is pictured on its launch pad in Tang Chung Ri, North Korea.
A closer look at the UNHA III rocket on its launch pad in Tang Chung Ri, North Korea.
A military vehicle participates in a parade in Pyongyang on April 15, 2012.
North Koreans wave flags in front of portraits of Kim Il Sung, left, and his son Kim Jong Il during celebrations to mark the 100th birth anniversary of Kim Il Sung in Pyongyang on April 16, 2012.
North Korean soldiers relax at the end of an official ceremony attended by leader Kim Jong Un at a stadium in Pyongyang on April 14, 2012.
Kim Jong Un applauds as he watches a military parade in Pyongyang on April 15, 2012.
A North Korean soldier stands on a balcony in Pyongyang on April 16, 2012.
North Korean soldiers march during a military parade in Pyongyang on April 15, 2012.
Soldiers board a bus outside a theater in Pyongyang on April 16, 2012.
North Korean performers sit below a screen showing images of leader Kim Jong Un in Pyongyang on April 16, 2012.
North Korean soldiers salute during a military parade in Pyongyang on April 15, 2012.
Kim Jong Un visits the Rungna People's Pleasure Ground, which is under construction in Pyongyang, in a photo released on July 3, 2012, by North Korea's official Korean Central News Agency.
North Korean soldiers listen to a speech during an official ceremony attended by leader Kim Jong Un at a stadium in Pyongyang on April 14, 2012.
Members of a North Korean military band gather following an official ceremony at the Kim Il Sung stadium in Pyongyang on April 14, 2012.
North Korean military personnel watch a performance in Pyongyang on April 16, 2012.
A North Korean controller is seen along the railway line between Pyongyang and North Pyongan province on April 8, 2012.
A North Korean military honor guard stands at attention at Pyongyang's airport during a diplomatic visit on May 2, 2001.
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