U.S. Secretary of State John Kerry said on Monday Syrian President Bashar Assad could avoid a military strike by turning over all his chemical weapons within a week but immediately made clear he was sure that would never happen.

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."

It did not appear that Kerry was making a serious offer to the Syrian government, which the United States accuses of using chemical weapons in an August 21 attack.

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.