British Prime Minister David Cameron says there is growing evidence the Syrian government has used chemical weapons against rebels, backing up a report by U.S. intelligence agencies.
Prime Minister Cameron on Friday called the development "very disturbing," adding that it would amount to a war crime and should represent a "red line" for the international community.
On Thursday, the White House said U.S. spy agencies believe with "varying degrees of confidence" the Syrian government has used sarin gas on a small scale against rebels trying to overthrow President Bashar al-Assad.
But the disclosure said the evidence was not conclusive enough to change U.S. decision-making on the issue.
Some U.S. lawmakers have been more vocal in calling for the U.S. to intervene militarily in the conflict, which has killed at least 70,000 people since it began in March 2011.
Meanwhile, fighting raged on in the capital, Damascus, on Friday. The Syrian Observatory for Human Rights says "fierce clashes" between rebels and government troops broke out in the Barzeh neighborhood in north Damascus.
The Britain-based group also said rebels and troops fought in Jobar district east of the capital, as well as parts of southern Damascus. The group, which gets its information from a network of activists and fighters on the ground, said at least 130 people died from the fighting on Thursday.