WASHINGTON (AFP) - President Barack Obama said that a Russian plan to secure Syria's chemical weapons could be a "significant breakthrough" but warned he had not taken US strikes off the table.
Mr Obama had intended to spend the day selling his plan to launch punitive military strikes against Bashar al-Assad's Damascus regime to skeptical US voters and lawmakers.
Instead, he found himself responding to a surprise Russian diplomatic initiative which would see Mr Assad's stockpile of banned chemical arms taken under international control.
The US leader, who faces a tough task winning Congressional approval for even a limited military action, expressed caution about the proposal but said it would be taken seriously.
And, in a series of television interviews, he insisted it had only come about because Mr Assad and his allies in Moscow could see the United States was serious about using force.
"I think what we're seeing is that a credible threat of a military strike from the United States, supported potentially by a number of other countries around the world, has given them pause and makes them consider whether or not they would make this move," he told NBC.
"And if they do, then this could potentially be a significant breakthrough. But we have to be skeptical because this is not how we've seen them operate over the last couple of years."
In separate interviews with several US broadcasters, Mr Obama said he had discussed the issue with Russia's President Vladimir Putin at last week's G20 summit in Saint Petersburg.
"If we can exhaust these diplomatic efforts and come up with an ... enforceable mechanism to deal with these chemical weapons in Syria, then I'm all for it," he told Fox.
Washington's European allies gave a similarly cautious welcome to the plan, and UN Secretary General Ban Ki Moon issued his own plea for a mission to secure and dispose of the weapons.
Russian Foreign Minister Sergei Lavrov had met his Syrian counterpart and urged Syria to "place chemical weapons under international control and then to have them destroyed."
Speaking in Moscow, Syria's Foreign Minster Walid al-Muallem welcomed the Russian move, though it was not immediately clear if a still defiant Assad would give his assent.
The rebels battling Mr Assad, who saw hope in the United States' threat to bomb the regime, denounced the idea as a plot by Mr Putin to protect Mr Assad.
Britain's Prime Minister David Cameron also expressed concern that the plan might be "a distraction tactic" but broadly welcomed it.
German Chancellor Angela Merkel described the Kremlin's proposal as "interesting" but added that she hoped it would be put into place quickly and not simply be used to "buy time." And France, the only Western ally to have offered to take part in a US-led strike, said Mr Assad must commit "without delay" to the elimination of his chemical arsenal.
United Nations leader Ban, meanwhile, called for the creation of UN supervised zones in Syria where the country's chemical weapons can be destroyed.
He told reporters he may propose the zones to the Security Council if UN inspectors confirm banned weapons were used and to overcome the council's "embarrassing paralysis" over Syria.
For his part, Mr Assad had warned in an interview with US television that the United States will "pay the price" if it attacks Syria.
While Mr Obama portrayed Russia's idea as a victory for Washington's policy of threatening military action, it still leaves him in a domestic political bind.
Having chosen to seek Congressional support for a limited US military strike against Syria, he could be defeated on his home turf.
Senate Majority Leader Harry Reid said he would delay a key procedural vote on authorizing force until after Mr Obama makes a national address on the issue on Tuesday.
"I wouldn't say I'm confident," Obama said of the prospect of his winning the impending votes.
"I'm confident that the members of Congress are taking this issue very seriously and they're doing their homework and I appreciate that."
Opposition is strong to a measure that is opposed by a majority of US voters, weary of war after drawn out, bloody and inconclusive American missions in Iraq and Afghanistan.
But US cruise missile destroyers are idling in the Eastern Mediterranean, preparing for what American officials described as a limited punitive strike.
According to US intelligence, on August 21 a chemical attack against rebel-held suburbs of Damascus killed more than 1,400 people, including 400 children gassed in their beds.
Other outside estimates set a lower but still high death toll, but Western capitals and the Arab League have condemned the alleged barrage as a war crime and blamed it on Mr Assad's regime.
Mr Obama has argued that an international military strike is necessary to defend the long-established international taboo against the use of such weapons.
The US lower house of the US Congress, the House of Representatives, is led by Republicans who oppose Obama's every move.
Some anti-war Democrats are also expected to oppose the motion, and the support of pro-war neo-conservatives in the Republican ranks may not be enough to push it through.
In his interview with NBC news, Mr Obama said: "There will be time during the course of the debates here in the United States for the international community, the Russians and the Syrians to work with us and say is there a way to resolve this."
Mr Obama has refused to rule out acting alone, with neither congressional nor international support, but defeat at home would be a blow to his credibility and strengthen Mr Assad's hand.
Fighting erupted in Syria in March 2011 when Mr Assad's forces launched a brutal crackdown on a popular revolt against his rule, and soon escalated into an all-out civil war. The UN estimates more than 100,000 have died.