British Prime Minister David Cameron said on Friday there was limited but growing evidence that Syria had committed a war crime by using chemical weapons.
In a cautious assessment mirroring that of U.S. President Barack Obama's administration, Cameron said the use of chemical weapons was a red line that should trigger greater pressure on President Bashar al-Assad.
"It is limited evidence but there is growing evidence that we have seen too of the use of chemical weapons, probably by the regime," Cameron told the BBC.
"It is extremely serious: this is a war crime ... We need to go on gathering this evidence and also to send a very clear warning to the Syrian regime about these appalling actions," he said.
The White House said on Thursday the Syrian government had probably used chemical weapons on a small scale in the country's civil war, but insisted Obama needed definitive proof before he would take action.
The disclosure created a quandary for Obama, who has set the use of chemical weapons as a "red line" that Assad must not cross. It triggered calls from some hawkish Washington lawmakers for a U.S. military response, which the president has resisted.
"I think what President Obama said was absolutely right - that this should form for the international community a red line for us to do more ... I have always been keen to do more," Cameron said.
But when he was asked if there could be troops deployed on the ground in Syria, Cameron said: "I don't want to see that and I don't think that is likely to happen, but I think we can step up the pressure on the regime, work with our partners, work with the opposition in order to bring about the right outcome."