Meeting for a summit in Paris, French President Nicolas Sarkozy and British Prime Minister David Cameron expressed support for a conference to form an international coalition dubbed the Friends of Syria next week in Tunis.
"We cannot accept that a dictator massacre his own people, but the revolution will not be brought from outside, it will rise from inside Syria, as it has done elsewhere," Sarkozy told a joint news conference.
"What is happening in Syria is appalling, for the government to be butchering and murdering its own people," Cameron said.
The two said France and Britain were working together to help the opposition, with Sarkozy urging anti-Assad forces to unite and be better organised.
"There are two things we can do to go further on Syria: first strengthen sanctions, not against the Syrian people but against the authorities, and second to think of what we can do to help the opposition to Bashar al-Assad's regime unite and become a credible alternative," Sarkozy said.
"We are ready to do more, but we are saying to all those who want democracy in Syria: 'Organise yourselves, gather together, tell us how we can help'," he said.
But both men said that conditions were not ripe in Syria for another Western military intervention like the one that tipped the balance in Libya.
"In Libya we had a UN Security Council that authorised force, we had an Arab League that wanted action to be taken, we had a clear opposition in Libya that was working on behalf of the whole of the country," Cameron said.
"With Syria we don't have all those circumstances in place but that doesn't mean we should stand back and just say there is nothing that can be done. We need to ... put the maximum pressure on Assad to stop the butchering."