Moscow and Washington are set to hold high-stakes talks on Russia's plan for Syria to eliminate its chemical weapons, with the United States urging President Bashar al-Assad to quickly declare the extent of his arms stockpile.
US Secretary of State John Kerry arrived in Geneva on Thursday for talks with Russian Foreign Minister Sergei Lavrov.
Officials from the two countries will pore over Moscow's plans to neutralise Syria's chemical arsenal - a last-minute proposal that led US President Barack Obama to put off plans for threatened US military action against the Syrian regime.
Washington wants to see if Assad is serious about putting his chemical weapons stockpile under international control - amid allegations the regime used sarin gas in an attack near Damascus on August 21 that killed more than 1400 people.
The aim of the talks is "to see if in fact we can test whether there is a credible and authentic way forward here, that the Russians mean what they say ... and probably more importantly that Assad means what he says," a senior US official said.
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"There are some specific things that we can ask for and see if they get delivered very quickly that will give us an early sense of whether there's reality here or not," the State Department official added.
They include that the Syrians "declare all of their stockpile quickly".
Kerry has alleged that Syria has about 1000 tonnes of various chemical agents.
Another senior US official said the US and Russia had expert knowledge in how to proceed in destroying such weapons, gleaned from destroying their own stocks of such internationally banned weapons.
The United States and its main backer of military strikes on Syria France 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.
Revealing details of the Russian proposal for the first time Thursday, 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 killed more than 110,000 people since March 2011.
The commander of the Free Syrian Army, Selim Idriss, said in a video posted on YouTube that the rebels categorically rejected the Russian initiative.
Idriss told world powers they should not "be satisfied only by removing the chemical weapon, which is the tool of a crime, but judge the author of the crime before the International Criminal Court."
The Syrian National Coalition opposition group said the plan is a "political manoeuvre aimed at buying time" for Assad and would be a "green light" to other regimes to use chemical weapons.
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."
Putin welcomed US willingness to consider the Moscow initiative, but 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.
The talks in Geneva were expected to last two to three days and also focus on revitalising efforts to organise a peace conference aimed at ending Syria's civil war.
As well as Lavrov, Kerry was due to meet UN-Arab League envoy Lakhdar Brahimi to discuss UN-backed efforts to bring the Assad regime to the table with the opposition rebels.
French Foreign Minister Laurent Fabius said on Thursday that a much-awaited report by UN inspectors into the attack will "probably" be published on Monday.
