U.N. investigators back in Syria to look into more chemical weapons claims

(CNN) -- U.N. inspectors returned to Syria on Wednesday to look into at least a half-dozen claims of chemical weapons use -- some allegedly by the regime, others allegedly by rebels.

The team has already confirmed the August 21 use of chemical warfare near Damascus but did not explicitly say who was responsible. According to U.S. estimates, that attack left more than 1,400 people dead.

The move came the same day a U.N. diplomat told CNN that three basic premises have been agreed upon for a U.N. Security Council resolution on Syria's chemical weapons disarmament.

Russia has basically agreed to the points, but "small issues" remain to be decided, the source said.

The deal could still fall apart.

According to the source, the three premises are: 1. Accountability for attacks on August 21; 2. Binding and enforceable language for measures against Syria in case of violations in the disarmament regime or the use of chemical weapons; 3. The council will remain seized of the matter, meaning that further discussion will be expedited.

The resolution would not authorize automatic use of force if Syria is said to be in violation, as was previously sought by the United States, the source said.

Among their tasks, investigators will try to figure out if chemical weapons were used in the northern city of Khan al-Asal in March. State-run media blamed rebels for the attack, which it said killed 25 people and injured more than 110 others. Syrian rebels, meanwhile, accused government forces of a chemical weapons attack on the rural Damascus suburb of Ateibeh.

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Security concerns delay U.N. inspectors

But the inspectors face a litany of challenges, like trying to find physical and forensic evidence from more than six months ago.

Then there's the security situation. During the inspectors' last visit, mortar shells landed near their hotel in Damascus, and their convoy came under fire while en route to a scene.

And just like their investigation into the August 21 attack, the inspectors will only try to determine whether chemical weapons were used during this trip, not who was responsible.

READ: U.S. official: Syrian CW list more complete than anticipated

CNN's Nick Paton Walsh, Holly Yan, Nick Thompson and Jomana Karadsheh contributed to this report.