The United States and Russia are to hold a third day of complex talks on Saturday, seeking to hammer out a deal on eliminating Syria's arsenal of chemical weapons, amid persistent differences.
During their second day of talks in Geneva on Friday, the US Secretary of State, John Kerry, and his Russian counterpart Sergei Lavrov shuttled with their delegations in and out of talks, breaking up after the midnight to allow the weary delegations to catch a few hours sleep.
The talks between the two sides are at a "pivotal point" and are continuing into Saturday, a US official told reporters.
"Both nations said they wanted to renew efforts to negotiate a peaceful end to the war in Syria", he said.
Teams of experts have been poring over a last-minute Russian initiative which led US President Barack Obama to back away from planned military strikes in response to an August chemical attack near Damascus that killed 1,429 people.
Washington blames President Bashar al-Assad regime for the attack, a charge the Syrian government has denied.
The US has estimated that Syria possesses about 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, without giving a figure.
"It's the same questions of what is it, where is it, how do we track it down, can we account for it, those are all part of the discussions," said a senior State Department official.
Washington and Moscow were also "working hard to find common ground" to get peace talks going in Geneva that would bring together Assad's regime and the opposition to end the war which erupted in March 2011, Kerry said on Friday.
In-depth coverage of escalating violence across Syria
In Washington, senior Obama administration officials said the United States did not expect a UN Security Council resolution formalising the deal to include potential use of military force. But officials said Obama retained that option.
At the United Nations, the Secreatry General, Ban Ki-moon, lashed out at Syrian President and said a UN inspectors' report into the incident would provide "overwhelming" confirmation that chemical weapons were used.
Assad had "carried out many crimes against humanity," Ban said, and insisted there had to be "accountability" once Syria's civil war was over.
The UN chief is expected to present a report on the use of chemical weapons in Syria to the Security Council on Monday morning, AP news agency reported.
Syria has filed documents at the United Nations seeking to join the international convention banning chemical weapons and said it now considers itself a full member.
But one of the issues still to be negotiated is the delay given to Syria to comply with the treaty's conventions and declare its full stockpile.
UN seeks more information
A UN spokesman said on Friday that the organisation has asked Syria for more information about its application, but he declined to say what was missing from the documents filed.
Washington has warned the regime that further steps will also be needed before military action would be off the table.
Fuelling concerns about Assad's sincerity, reports emerged on 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.
Syria's opposition National Coalition also said it was "deeply sceptical" of the government's move and urged a tough UN resolution to enforce the measure.