'Prove it,' US tells Assad on chemical arms vow

But the long-time Syrian leader cast fresh doubt on how committed he is to a hastily-conceived plan to secure Syria's poison gas stocks, by demanding that Washington first drop its threat of military action against his regime.

"When we see that the United States truly desires stability in our region and stops threatening and seeking to invade, as well as stops arms supplies to terrorists then we can believe that we can follow through with the necessary processes," Assad told Russian television.

"Syria is handing over chemical weapons under international control because of Russia," he said. "US threats have not affected the decision."

Top US and Russian diplomats and weapons experts launched high-stakes talks late Thursday to pore over the details of the Russian plan, and were to huddle again in their Geneva hotel early Friday.

But US Secretary of State John Kerry warned Assad that "the words of the Syrian regime in our judgement are simply not enough".

"Expectations are high. They are high for the United States, perhaps even more so for Russia to deliver on the promise of this moment. This is not a game," Kerry said.

Any deal to bring Syria's chemical weapons stockpile under international control "has to be credible. It has to be timely and implemented in a timely fashion," he said.

In a concrete move towards disarmament, Syria on Thursday filed documents at the United Nations seeking to join the international convention banning chemical weapons.

Russian Foreign Minister Sergei Lavrov said he believed: "The solution of this problem makes unnecessary any strikes on Syria".

Pending the talks, US President Barack Obama has put on hold plans for limited military strikes against the Syrian regime to disable its chemical weapons capability, but US officials have repeatedly stressed that the option still remains on the table.

Lavrov told Kerry, speaking through a translator, that "I hope we will achieve all the successes".

But Kerry quipped: "You want me to take your word for it? It's a little early for that".

The two men and their delegations met for about an hour, before Lavrov and Kerry then held a smaller dinner together. US officials described the talks as "comprehensive" and the atmosphere as "constructive."

"They agreed on a shared goal of achieving a framework for the path forward," a senior State Department official said, asking not to be named.

Washington alleges that some 1,400 people died in a chemical attack on August 21 and was rallying support for a military response when the Russian proposal emerged.

The United States and France, its main backer of military strikes on Syria, have warned they will not allow the chemical weapons plan to become a delaying tactic in Syria's brutal war, saying the threat of military force remains on the table.

"All of this should, if everyone is aware of their responsibilities, allow for the end of chemical weapons in Syria and for us to find a political solution, but France is keeping up pressure," President Francois Hollande told journalists.

Revealing details of the Russian proposal, the daily Kommersant said Moscow had given Washington a four-step plan for the weapons handover.

Quoting a Russian diplomatic source, Kommersant said the plan would see Damascus 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.

Syria's opposition has denounced the plan, warning it will only lead to more deaths in a conflict that has already claimed more than 110,000 lives since March 2011.

Russian President Vladimir Putin meanwhile made an unusual personal appeal to the American people to reject military action, in an opinion piece in the New York Times.

"A strike would increase violence and unleash a new wave of terrorism," Putin wrote. "It could throw the entire system of international law and order out of balance."

He warned any strikes without the approval of the United Nations Security Council, where Moscow wields a veto, would destroy the credibility of the world body.

Russia is a traditional ally of Assad, and Moscow, backed by China, has blocked any attempt to sanction his regime through the United Nations.

French Foreign Minister Laurent Fabius willl visit China on Sunday to discuss the Syria plan, before travelling to meet with Lavrov.

On Friday, Hollande will meet the foreign ministers of Saudi Arabia, Jordan and the United Arab Emirates, his office said.

Kerry also met Thursday with UN-Arab League envoy Lakhdar Brahimi to discuss UN-backed efforts to bring the Assad regime to the negotiating table with the rebels.