BBC News - Syria: Cameron and Obama threaten 'serious response': "The UK and the US have threatened a "serious response" if it emerges Syria used chemical weapons last week.
Prime Minister David Cameron and President Barack Obama spoke on the phone for 40 minutes on Saturday, Mr Cameron's office said.
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."
