* Proposal apparently began as off-the-cuff remark by Kerry
* Russian initiative backed by China, Iran
* Rebels dismayed by proposal that would avert strikes
By Mark John and Dominic Evans
PARIS/BEIRUT, Sept 10 (Reuters) - France is to put forward a U.N. Security Council draft resolution for Syria to give up its chemical weapons, quickly turning a Russian idea into a full-blown diplomatic proposal that could avert Western military strikes.
The French draft would threaten "extremely serious" consequences if Syria violates the conditions for giving up its chemical weapons, Foreign Minister Laurent Fabius said on Tuesday.
Syria's rebels reacted with deep dismay on Tuesday to the proposal, which would avert Western military action to punish President Bashar al-Assad's forces for a poison gas attack that killed hundreds of people in a Damascus suburb last month.
Moscow said it was working on a concrete proposal for Syria to give up its poison gas stockpiles, a day after floating the idea in what U.S. President Barack Obama said could be a "breakthrough".
The Russian proposal, which apparently began life as an off-the-cuff remark by U.S. Secretary of State John Kerry, offers Obama a way out of ordering unpopular strikes, days before contentious votes in Congress seeking authorisation to use force.
With veto-wielding China also backing the proposal, it could be the rare initiative to unite global powers whose divisions so far have blocked the Security Council from acting.
While diplomacy took that extraordinary turn, the war that has already killed more than 100,000 people and driven millions from their homes ground on, with Assad's forces launching an offensive to take back a town north of Damascus.
Washington and Paris have threatened to carry out strikes to punish Assad for the Aug. 21 poison gas attack on Damascus suburbs, which they say Syrian government forces carried out.
But after 12 years of wars in Iraq and Afghanistan, Obama has had a hard time winning over the public or members of Congress. Britain quit the coalition threatening force after Prime Minister David Cameron lost a vote in parliament.
Moscow unveiled its proposal on Monday after Kerry, speaking in London, said the only way to halt strikes would be for Assad to give up his chemical weapons arsenal.
Russian Foreign Minister Sergei Lavrov quickly called for Assad's government to do just that, and Assad's foreign minister, in Moscow on a visit, agreed. On Tuesday, Lavrov said the proposal was not entirely Russian but had grown out of contacts with the United States.
Obama told CNN: "It's possible that we can get a breakthrough," although he said there was a risk that it was a further stalling tactic by Assad.
"We're going to run this to ground," he said. "John Kerry and the rest of my national security team will engage with the Russians and the international community to see, can we arrive at something that is enforceable and serious."
Such wavering from the West was dismaying for the Syrian opposition, which has been pleading for two and a half years for support from Western governments that long ago demanded Assad's removal from power but have declined to intervene.
The Russian proposal "fails to hold the Assad regime responsible for the killing of innocents," the Syrian National Coalition said on Tuesday.
The proposal "is a political manoeuvre which will lead to pointless procrastination and will cause more death and destruction to the people of Syria, and further threats to the countries and people of the region."
"Crimes against humanity" could not be dealt with through "political concessions, or by handing over the means by which they were carried out," it said.
Syria is not a party to international treaties which ban the stockpiling of chemical weapons, although it is a party to the Geneva conventions that forbid using them in warfare. Syria has tried to avoid confirming whether it possesses poison gas, while denying it has used it.
Western countries believe Syria has a vast undeclared arsenal of chemical arms. Sending inspectors to destroy it would be difficult even in peace time and extraordinarily complicated in the midst of a war.
The two main precedents are ominous: U.N. inspectors dismantled the chemical arsenal of Iraqi leader Saddam Hussein in the 1990s but left enough doubt that suspicion he still had such weapons was the basis for a U.S.-led invasion in 2003.
Libyan leader Muammar Gaddafi was rehabilitated by the West after agreeing to give up his banned weapons, only to be overthrown with NATO help in 2011.
Assad's government says the chemical attack was the work of rebels trying to win Western military support, a scenario that Washington and its allies say is not credible.
Human Rights Watch, the New York-based watchdog, said evidence strongly suggested Syrian government forces were behind the attack. It said in a report that the type of rockets and launchers used in the attacks suggested weapon systems in the possession only of government forces.
In Congress, Democratic Senate leader Harry Reid pushed back a Senate test vote on possible U.S. strikes that had been scheduled for Wednesday as lawmakers evaluate the Russian plan.
The vote is still expected this week, and a more contentious vote would later be held in the Republican-controlled House of Representatives.
The dramatic diplomatic twist in weeks of high-tension international wrangling came when Kerry was asked by a reporter during a visit to London whether there was anything Assad's government could do or offer to stop a U.S. military strike.
"Sure. He could turn over every single bit of his chemical weapons to the international community in the next week - turn it over, all of it without delay and allow the full and total accounting. But he isn't about to do it and it can't be done."
The State Department later said Kerry had been making a rhetorical argument, not an explicit proposal.
But less than five hours later, Russian Foreign Minister Lavrov said he had put the idea to his visiting Syrian counterpart during talks in Moscow. Walid al-Moualem said Damascus welcomed the Russian initiative - while not spelling out whether Syria would, or even could, comply.
Assad's main Middle East backer Iran said it supported Russia's offer.
U.N. Secretary-General Ban Ki-moon also backed the ideas in the proposal, saying he was considering proposing something similar to the Security Council.
Rebels were livid. Qassim Saadeddine, a rebel commander in northern Syria and a spokesman for the Supreme Military Council of Assad's opponents, said: "It is a trap and deceitful manoeuvre by the Damascus regime and will do nothing to help the situation.
"They have tons of weapons hidden that would be nearly impossible for international inspectors to find."
Inside Syria, government forces launched an offensive to wrest back control of a historic Christian town north of Damascus on Monday, activists said. In the past six days, the town of Maaloula has already changed hands three times between Assad's forces and rebels, some of whom are linked to al Qaeda. (Writing by Peter Graff; Editing by Giles Elgood)