Russia's proposal to stave off threatened US strikes Syria through a handover of chemical weapons received a cautious welcome Tuesday even from backers of military action, with President Barack Obama describing the idea as a "potential breakthrough".

Only the opposition fighting the regime of President Bashar al-Assad for control of Syria openly denounced the Russian idea, describing it as a political manoeuvre that will waste time and cause more deaths.

Russian Foreign Minister Sergei Lavrov the day earlier had announced during a hastily called news briefing after talks with Syrian counterpart Walid al-Muallem a plan for Damascus to "place chemical weapons under international control and then to have them destroyed."

Speaking in Moscow, Muallem said he "welcomed" the Russian move without immediately accepting the proposal. It was not immediately clear if a still defiant Assad would agree to the measure.

Lavrov said that the plan could avert threatened military strikes by the United States after a chemical weapons attack outside Damascus on August 21 which the West believes was carried out by the regime.

Obama warned Monday he had not taken military strikes off the table but, in agreeing to consider the Russian initiative, he effectively pushed back the timetable for possible action.

"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 television.

"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."

Obama, who faces a tough task winning Congressional approval for even a limited military action, admitted that US lawmakers were not close to voting on the issue.

"I don't anticipate that you would see a succession of votes this week or anytime in the immediate future," he told ABC news.

The Russian plan came in apparent response to remarks Monday by Secretary of State John Kerry who said that to avoid military action, Assad could turn over his chemical weapons to the international community.

"Turn it over, all of it, without delay and allow a full and total accounting for that," Kerry told reporters in London. "But he isn't about to do it and it can't be done."

But Obama said he had discussed the issue with Russian President Vladimir Putin at last week's G20 summit in Saint Petersburg.

The rebels battling Assad, who hoped to see US missiles rain down on the regime, denounced the idea as a plot by the Kremlin to protect Assad.

"The proposal of Lavrov is a political manoeuvre and is part of useless procrastination that will only result in more deaths and destruction for the Syrian people," said an opposition statement.

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."

France, the only Western ally to have offered to take part in a US-led strike, said Assad must commit "without delay" to the elimination of his chemical arsenal.

The foreign ministry of China, which has backed Russia's stance throughout the crisis, predictably said that "we welcome and support the Russian side's suggestion.'

Iran, an ally of both Syria and Russia, "favourably welcomes" the Russian plan, foreign ministry spokeswoman Marzieh Afgham said.

For his part, Assad warned earlier in an interview with US television that the United States will "pay the price" if it attacks Syria.

While Obama portrayed Russia's idea as a victory for Washington's policy of threatening military action, it still leaves him in a 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 authorising force until after 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.

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.

US-based rights group Human Rights Watch said in a statement early Tuesday that all available evidence "strongly suggested" the Syrian government forces were responsible for the attack.

Human Rights Watch issued its findings in a 22-page report after analysing witness accounts of the rocket attacks, the physical remnants of weapons used and the symptoms exhibited by the victims.

The crisis in Syria flared after Assad's forces launched a brutal crackdown on peaceful anti-regime protests that began in March 2011, and eventually degenerated into an all-out civil war that has killed more than 100,000 people, according to the UN.

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