UN chief Ban Ki-moon on Friday meanwhile accused Syrian President Bashar al-Assad of crimes against humanity as the talks aimed at dismantling Syria's chemical weapons arsenal hit a pivotal point.
During their second day of talks in Geneva on Friday, US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov shuttled with their delegations in and out of talks, breaking up around 2am to allow the weary delegations to catch a few hours sleep.
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, which Washington blames on the regime and says killed about 1 400 people.
At the United Nations, Ban lashed out at Assad and said a UN inspectors' report into the incident would provide "overwhelming" confirmation that chemical weapons were used.
The Syrian leader President Bashar al-Assad had "carried out many crimes against humanity," Ban said, and insisted there had to be "accountability" once Syria's civil war is over.
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 US 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, 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.
"We're obviously at a pivotal point."
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 Friday.
He will meet again with Lavrov on the sidelines of the UN General Assembly in New York later this month, with the hope of setting a date for the peace conference, he said.
Taking a breather from the intense diplomacy, Kerry set off on a brisk walk from his luxury hotel to Lake Geneva early Saturday, taking a few close aides and his security staff with him.
Arriving back at the hotel, he told reporters "we're working hard" as he rushed through the lobby and back to his suite.
Following the Geneva talks, Kerry will fly to Israel to brief Israeli Prime Minister Benjamin Netanyahu on Sunday.
As the diplomatic drive intensifies, 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 filed documents at the UN 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.
A UN spokesperson said Friday 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 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, but reports say the plan calls for Damascus to join the Organisation for the Prohibition of Chemical Weapons (OPCW), declare the locations of its chemical arms, allow OPCW inspectors access and finally arrange for destruction of the arsenal.
A spokesperson for the Hague-based OPCW said it will meet next week to examine Syria's request.
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.
But senior White House officials said the United States concedes that Russia will not allow the UN Security Council to authorize military strikes against the Syrian regime.
Speaking on condition of anonymity, the officials said a UN report due on Monday would strengthen the case that Bashar al-Assad used chemical weapons against his own people.
But they admitted that this, while weakening Moscow's claim that anti-government rebels fired the poison gas, would not be enough to swing the Kremlin behind military action. – Sapa-AFP