David Cameron said he has not ruled out stepping up action against the Syrian regime after growing evidence they were using chemical weapons, according to Reuters.
Cameron told NPR radio: "Certainly we haven't ruled out stepping up action in response to what seems to be happening on the ground.
"The evidence is growing; the lack of room for doubt is shrinking, and I think this is extremely serious."
The Prime Minister said he would raise the issue with US President Barack Obama at their meeting today.
More top news
Demonstrators have blockaded the entrance tunnel to Heathrow Airport in apparent protest at plans to build a third runway.
A man was left red-faced after neighbours reported shouting and a woman screaming - which turned out to be his efforts to kill a spider.
A man who shot and killed his wife before posting a photo of her body - and a confession - to Facebook has been found guilty of murder.