US Secretary of State John Kerry said on Monday Syrian President Bashar al-Assad could avoid a military strike by turning over all his chemical weapons within a week but immediately made it 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 whether there was anything Assad's government could do or offer to stop any attack, Kerry said: "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 US television network CBS, Assad said the United States would be going against its own interests if it got involved in Syria, warning of repercussions.

Kerry said the control of chemical weapons in Syria was limited to Assad, Assad's brother Maher and an unnamed general.

Kerry said he was confident of the evidence that the United States and its allies have presented to support their case that Assad's forces used chemical weapons, though he said he understood concerns, given the discord over the 2003 Iraq war.

Speaking at a news briefing in London with British Foreign Secretary William Hague, Kerry said that doing nothing in the face of such evidence 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.

He also stressed the relationship between Britain and the United States was as strong as ever despite the British parliament having decided not to join military action against Syria.

"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 vote."

Kerry said while in London he had held talks with Palestinian President Mahmoud Abbas which were "productive and information" but did not give any further details.-Reuters