Britain has warned that assurances by Syria that it is ready to give up its chemical weapons must be treated with "great caution" as international talks were getting under way to test a Russian proposal to put president Bashar Assad's deadly arsenal under international control.
US Secretary of State John Kerry is in Geneva with a team of experts at the start of two days of discussions with his Russian counterpart Sergei Lavrov aimed at averting military action.
The Americans want a rapid agreement on a set of principles for carrying out the hugely complicated process of identifying and isolating Syria's chemical stockpiles so they can be destroyed in the midst of a war zone.
President Barack Obama, who put US missile strikes on hold following Moscow's surprise diplomatic initiative on Monday, has warned that it must not be a delaying tactic and US officials say they will know relatively quickly if the Russians are trying to stall.
"Our goal here is to test the seriousness of this proposal, to talk about the specifics of how this would get done, what are the mechanics of identifying, verifying, securing and ultimately destroying the chemical weapons," said State Department spokeswoman Jen Psaki.
The US hopes that an agreement with the Russians - Assad's most important ally - could be incorporated into a binding new UN Security Council resolution requiring the Assad regime to surrender its nuclear weapons.
The five permanent Security Council members - the US, Russia, Britain, France and China - last held preliminary talks to consider a draft resolution prepared by the French. Submitted under Chapter VII of the UN charter, which makes it enforceable militarily, it warns of "very serious consequences" if Syria does not comply.
The Russians, however, have strongly rejected the idea of a Chapter VII resolution and Foreign Secretary William Hague has indicated there could be some flexibility on the issue.
Speaking in the House of Commons, he refused to be drawn on whether Britain wanted a Chapter VII resolution, saying, simply that it must be "binding" with a requirement for Syria to give up its weapons "within a specific time frame".
"Given their track record, any commitment made by the Syrian regime must be treated with great caution. This is a regime that has lied for years about possessing chemical weapons, that still denies it has used them, and that refused for four months to allow UN inspectors into Syria," he said. "The United Kingdom will make every effort to negotiate an enforceable agreement that credibly, reliably, and promptly places the regime's chemical weapons stocks under international control for destruction," he said.