The United States and Russia on Saturday began a third day of talks to hammer out an agreement on eliminating Syria's chemical weapons, as the UN chief voiced doubt the regime would respect any deal.
After discussions stretching into the early hours of Saturday, US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov resumed talks amid cautious hopes progress was being made.
Teams of experts have been poring over a surprise Russian initiative which led US President Barack Obama to put on hold planned military strikes in response to an August chemical attack near Damascus, which Washington blames on the regime and says killed about 1,400 people.
The two sides "made progress in coming closer to agreement on the scope of the chemical weapons stockpile", a senior US administration official said.
The United States has estimated that Syria possesses around 1,000 metric tonnes of various chemical agents, including mustard and sarin gas, sulfur and VX.
The Russian estimates had been initially much lower, the official said.
But differences remained Saturday, notably over what deadline should be set for the regime of Syrian President Bashar al-Assad to declare his country's chemical arms stockpile, and the wording of a UN resolution.
"We're obviously at a pivotal point," a State Department official said late Friday.
The United States and its allies are also working to draft a separate UN resolution to back any plan drawn up in Geneva.
Senior White House officials admitted Friday that Russia, which holds a veto at the UN Security Council, would not agree to a UN resolution that could threaten military strikes against the Syrian regime if it fails to comply with the deal.
While Obama would still keep the threat of US military action on the table, other ways of enforcing a UN resolution, including sanctions, were being examined, the officials said.
UN chief Ban Ki-moon meanwhile joined international scepticism over whether Assad would respect an international deal.
Assad seemed to have responded "positively" so far, Ban told France 24, "but at the same time I sense in the international community some sense of scepticism."
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"Yes, I also share" the doubts, he added.
Ban has also accused Assad of crimes against humanity as he said a UN inspectors' report into the incident would provide "overwhelming" confirmation that chemical weapons were used.
Assad has "carried out many crimes against humanity", Ban said, insisting there had to be "accountability" once Syria's civil war is over.
Washington and Moscow are also hoping to revive plans for peace talks in Geneva that would bring together Assad's regime and the opposition to agree a political transition to end the war that erupted in March 2011.
Kerry and Lavrov will meet again in a few weeks on the sidelines of the UN General Assembly in New York, with the hope of setting a date for the stalled peace conference.
As the diplomatic drive intensifies, Kerry will fly to Israel to brief Prime Minister Benjamin Netanyahu on Sunday.
He will then travel to Paris to meet French Foreign Minister Laurent Fabius and British Foreign Secretary William Hague as well as the Saudi Foreign Minister Saud al-Faisal.
Syria has meanwhile filed documents at the United Nations seeking to join the international convention banning chemical weapons and said it now considers itself a full member.
A UN spokesman said Friday that the organisation asked Syria for more information about its application, but he declined to say what was missing from the documents filed.
Fuelling further concerns about Assad's sincerity, reports emerged Friday that a secret Syrian military unit was scattering the chemical weapons stockpile around the country.
The unit was given responsibility to shift the arsenal of poison gases and munitions to different locations across Syria, the Wall Street Journal reported, citing US and Middle Eastern officials.
Russia has not revealed many details of its plan to bring Syria's chemical weapons under international control.
But reports say the proposals call for Damascus to join the Organisation for the Prohibition of Chemical Weapons (OPCW), declare the locations of its chemical arms, allow access to OPCW inspectors and finally arrange for destruction of the arsenal.
A spokesman for the Hague-based OPCW said it will meet next week to examine Syria's request.