By Arshad Mohammed and Andrew Osborn

LONDON (Reuters) - Syrian President Bashar al-Assad could avoid a U.S. military strike by surrendering all his chemical weapons within a week, U.S. Secretary of State John Kerry said on Monday, but immediately made clear he was not making a serious offer.

President Barack Obama is seeking support from Congress for punitive military action against Syria over a suspected chemical weapons attack in a civil war that the United Nations says has killed at least 100,000 people.

When asked by a reporter in London whether there was anything Assad's government could do or offer to stop a military strike, Kerry answered:

"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 (of it), 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 about the impossibility of Assad turning over chemical weapons, which Assad denies his forces used in the August 21 poison gas attack.

In an interview with U.S. television network CBS, Assad said the United States would be going against its own interests if it got involved in Syria, warning of repercussions.

Russian President Vladimir Putin, Assad's only big-power supporter, says opponents of Assad staged the attack to provoke U.S.-led military intervention, an allegation Kerry dismissed out of hand on Monday.

Putin's foreign minister, Sergei Lavrov, met Assad's foreign minister, Walid al-Moualem, in Moscow on Monday and the two urged Washington to concentrate on convening a Syrian peace conference rather than on military action.

Moualem suggested the chemical attack was a pretext to trigger military intervention and asked whether Obama was backing "terrorists" - an allusion to radical Islamists, who are prominent in the ranks of rebels fighting to topple Assad.

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WAR IN SYRIA

Kerry said he was confident of the evidence that the United States and its allies had presented to support their case that Assad's forces used poison gas, though he said he understood skepticism lingering from the 2003 Iraq war - in which cited intelligence about weapons of mass destruction in the hands of Iraqi dictator Saddam Hussein was later proven wrong.

He avoided directly answering a question on whether the United States had evidence directly linking Assad to the alleged August 21 chemical weapons attack, but said such weapons were controlled by only three people in Syria: Assad, his brother Maher, and an unnamed general.

Kerry, a former lawyer, said he had successfully prosecuted people with less evidence and warned that doing nothing was worse than doing something, saying inaction would come back to haunt the United States and its allies.

"If you want to send Iran and Hezbollah and Assad a congratulatory message: 'You guys can do what you want,' you'd say: 'Don't do anything.'

"We believe that is dangerous and we will face this down the road in some more significant way if we're not prepared to take ... a stand now," Kerry said.

When asked about the CBS interview with Assad, British Foreign Secretary William Hague cautioned against giving too much weight to the Syrian president's words.

"We mustn't fall into the trap of attaching too much credibility to the words of a leader, President Assad, who has presided over so many war crimes and crimes against humanity (and) shown such a murderous disregard for the welfare of his own people," Hague said. "So let's not fall into the trap of believing every word that ... comes out of such a man."

Kerry stressed that ties between Britain and the United States were as strong as ever despite the British parliament voting not to endorse military action against Syria, prompting a government decision not to take part.

"The relationship between the United States and the UK has often been described as special, essential and it has been described thus because it is," Kerry said. "The bond ... is bigger than one (parliamentary) vote."

While in London, Kerry said he had held talks with Palestinian President Mahmoud Abbas that he described as "productive", but did not give further details.

(Writing by Guy Faulconbridge and Belinda Goldsmith; Editing by Stephen Addison and Mark Heinrich)