John Kerry, the US secretary of state, yesterday strengthened the case for military action against Bashar al-Assad’s regime after announcing that tests conducted on gas attacks in Syria had proved positive for the nerve agent sarin.

As he kicked off a week of intense lobbying, Mr Kerry urged Congress to consider the security of US allies Israel and Jordon when they vote next week over whether to authorise force against Syria after Barack Obama unexpectedly delayed air strikes.

The president’s decision on Saturday to ask Congress to support the move caught even his advisers by surprise. The US had indicated that Washington would stick to its timetable for a weekend strike after the UK voted on Thursday to reject involvement in military action.