The White House gave a cautious welcome on Monday to a Russian proposal for Syria to hand over its chemical weapons, opening up the first real chance of a political settlement to the crisis since hundreds of civilians died in an attack on a Damascus suburb last month.
US deputy national security adviser Tony Blinken said "it would be terrific" if Syria followed through on a reported offer by its foreign minister to place chemical stockpiles under the control of international observers. But he nevertheless expressed scepticism whether it would do so. "Unfortunately, the track record to date does not inspire a lot of confidence," Blinken said.
The White House said it would now work with the Russians to explore the deal proposed earlier on Monday by foreign minister Sergey Lavrov, after an apparent off-the-cuff remark by US secretary of state John Kerry. The administration stressed that these discussions would take place "in parallel" with continued efforts in Washington to persuade US lawmakers to authorise the use of military force against Syria.
The diplomatic scramble began in London when Kerry suggested that the only way for Syria to avoid the threat of a US attack would be for it to hand over all its chemical weapons within a week.
At first, the significance of the remarks were downplayed by the Department of State, which said he had been speaking "rhetorically", but Kerry's language was immediately seized on by Lavrov, who raised the prospect of international observers supervising such a handover.
"If the establishment of international control over chemical weapons in that country would allow avoiding strikes, we will immediately start working with Damascus," Lavrov said.
"We are calling on the Syrian leadership to not only agree on placing chemical weapons storage sites under international control, but also on its subsequent destruction and fully joining the treaty on prohibition of chemical weapons," Lavrov said after a meeting with his Syrian counterpart, Walid al-Moallem.
Whether intentional or not, Kerry's comments opened up a chance to defuse the crisis at a moment when Barack Obama was already struggling to persuade Congress of the need for US intervention. Kerry later spoke to Lavrov by phone and Washington scrambled to place its own spin on the unexpected breakthrough.
Obama was due to give a round of TV interviews in the US on Monday evening before delivering a direct address to the nation on Tuesday.
A key legislative ally, Senator Dianne Feinstein of California, the chairwoman of the Senate intelligence committee, said that she would welcomed a move by Syria to put its chemical weapons beyond use.
"I believe that Russia can be most effective in encouraging the Syrian president to stop any use of chemical weapons and place all his chemical munitions, as well as storage facilities, under United Nationscontrol until they can be destroyed," Feinstein said.
The former US secretary of state, Hillary Clinton, speaking after a hastily arranged meeting with Obama, said the move could be an "important step".
White House spokesman Jay Carney insisted that the offer by Russia and Syria had only come about because of "sustained pressure" from the US.
"It is our position and has been for some time that the Syrian regime should not use and also not possess stockpiles of chemical weapons, and we would welcome any proposals that would result in the international control and destruction of that chemical weapons stockpile," he said in a press conference.
"There is no question that we have seen some indications of an acceptance of this proposal [from the Syrians], but this is a very early stage and we approach this with scepticism," added Carney.
Moallem, who is visiting Moscow, suggested the deal could be acceptable in Damascus.
"Syria welcomes the Russian proposal out of concern for the lives of the Syrian people, the security of our country and because it believes in the wisdom of the Russian leadership that seeks to avert American aggression against our people," Moallem said in a statement.
The proposal was welcomed by the UN and a number of European governments. UN secretary general Ban Ki-moon said he would propose the security council unite and vote on an immediate chemical weapons transfer, placing weapons and chemical precursors in a safe place within Syria for international destruction.
Earlier, Ban said that he hoped that a forthcoming report by UN inspectors on the 21 August attack on a rebel-held area east of Damascus called Ghouta, which the US says killed more than 1,400 people, would spur the international community into action.
"Two and half years of conflict in Syria have produced only embarrassing paralysis in the security council," Ban said at a press conference.
The French government has said it would wait for the UN report, being prepared by a Swedish scientist, Åke Sellström, before making a final decision on taking part in military action.
The Sellström report is unlikely to come before the end of this week, diplomatic sources said. The samples brought back from a two-week visit are being studied in four European laboratories, to ensure that the result is conclusive.
In the British parliament, David Cameron responded positively but cautiously to Russia's move, saying if it was a genuine offer, it should be regarded as a big step forward.
Downing Street initially indicated that the Kerry proposal was not serious, pointing out that the idea had not been raised during the lengthy discussion on Syria at the G20 dinner in St Petersburg. They added that the focus should be on Assad's record with chemical weapons.
But in a Commons debate on the G20 and Syria, Cameron said it would be "hugely welcome" if the Assad regime were to hand over its chemical weapons stockpile.
Susan Rice, the US national security adviser, said that "even greater barbarism" would follow if the US did not take military action against Assad. "The decision our nation makes in the coming days is being watching in capitols around the world, especially in Teheran or Pyongyang," Rice told an audience at the New America Foundation in Washington on Monday.
Rice, the former US ambassador to the UN, did not address Russia's offer for Assad to relinquish his chemical stockpiles.
Additional reporting by Spencer Ackerman in Washington and Patrick Wintour in London