U.N. chief Ban Ki-moon Friday accused Bashar al-Assad of crimes against humanity as complex, high-stakes talks between the US and Russia on dismantling Syria's chemical weapons arsenal reached a pivotal point, stretching late into the night.

   U.S. Secretary of State John Kerry and his Russian counterpart Sergei Lavrov were holding a second day of talks in Geneva to hammer out the details of a Russian plan to bring the weapons under international control that emerged this week.

   All day the top diplomats plus their delegations shuttled in and out of meetings, breaking up into smaller technical teams, and then rushing back to consult with their governments. As both sides hailed the talks as "constructive," they confirmed the talks were looking likely to stretch into Saturday.

   Teams of experts have been poring over a last-minute Russian initiative which caused U.S. 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 and end the war which erupted in March 2011, Kerry said earlier.

   He would 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.

   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 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 time delay given to Syria to comply with the treaty's conventions and declare its full stockpile.

   A UN spokesman 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 Russian media report that it calls for a four-step process for the weapons handover.

   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 spokesman 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.

   "It is vital the threat of force stays on the table. For a (UN Security Council) resolution to be anything other than a get-out-of-jail-free card for the regime, it must be enforceable under Chapter 7," allowing military action,

it said in a statement.

   But it would appear, according to White House officials, that an explicit threat of force may not be included in the draft of a UN resolution, in favor of other means of pressure such as sanctions.

   They also told reporters that they expected to know within two weeks if Syria was serious giving up its chemical arms.

   The French presidency said meanwhile that France, Saudi Arabia, the United Arab Emirates and Jordan had agreed to give more help to the Syrian opposition in its battle against Assad. (AFP)