The UK and the US have pledged to help strengthen the moderate opposition in Syria and create a transitional body to replace President Bashar al-Assad.
British Prime Minister David Cameron said in Washington that there was an "urgent window of opportunity before the worst fears are realised".
"There's no more urgent international task than this. We need to get Syrians to the table to agree a transitional government that can win the consent of all of the Syrian people," he said.
"There will be no political progress unless the opposition is able to withstand the onslaught and put pressure on Assad so he knows there is no military victory."
"I do believe that there is more we can do alongside technical advice, assistance, help, in order to shape them, in order to work with them," Mr Cameron said, stressing that no decision had yet been made regarding the supply of weapons to the Syrian opposition.