Obama and Cameron Threaten 'Serious Response' if Use of Chemical Weapons in Syria is Confirmed

Opposition activists claims hundreds were killed in recent chemical attack outside Damascus

The UK and the US have threatened a "serious response" if it emerges Syria used chemical weapons last week, Downing Street has said.

UK Prime Minister David Cameron and US President Barack Obama spoke on the phone for 40 minutes on Saturday.

Both were "gravely concerned" by "increasing signs that this was a significant chemical weapons attack carried out by the Syrian regime".

The Syrian regime and opposition have accused each other over the attacks.