The US Secretary of State John Kerry says Syria has the ability to prevent a possible military attack by American forces.

Kerry says a strike wouldn't go ahead if Syrian President Bashar al-Assad handed over all his chemical weapons within the next week.

Although he did concede that was unlikely to happen.

John Kerry held talks in London this morning with the British Foreign Secretary William Hague and insisted the international community has to take action.

Hague told a news conference at the Foreign Office that the UK supported "mustering a strong international response" to Assad's alleged use of chemical weapons that killed more than 1,400 civilians in Damascus.

Kerry stressed the importance of the "special relationship" between Britain and the US and said the two countries were "enormously tied together".

The risk of not acting was greater than the risk of acting, he said.