There is "no sign" that Russia's support for the Syrian regime is waning despite continuing brutal repression, David Cameron said.
The Prime Minister said he had discussed the crisis during a telephone call with newly elected president Vladimir Putin, adding: "I did not sense any sign of a shift."
Giving evidence to the Commons liaison committee, Mr Cameron also warned that those giving "succour" to president Bashar Assad were risking the chaos of a popular revolution.
"What is the best path to end the violence in Syria?" he said. "A transition at the top of that regime with Assad going." The premier added: "Having him stay is a cause of instability."
Mr Cameron said: "I think that the idea of transition at the top is actually possibly a better outcome, a less bloody outcome than a transition at the bottom. Those who are giving succour to President Assad, who think that somehow clinging to him ensures stability in Syria, are absolutely wrong. They may run out of time when actually the revolution from the bottom may help to unseat him rather than transition at the top."
The premier played down the prospect that the arms embargo could be dropped in order to assist the opposition.
And he also insisted that those opposed to Assad had to show more unity.
"What would be welcome (is) if there was a clearer establishment of who the Syrian opposition are and a clearer sense that they are genuinely representative of a future for Syria that would be democratic and open and tolerant of minorities and the rest.
"I think if you compare it with the situation in Libya I think the National Transitional Council (NTC) did a good job at bringing opposition elements together and giving the world someone to talk to and someone to work with.
"I think in the Syrian national council we now have a body that we can see as a legitimate body, but frankly I think there is more work that we need to understand the various elements of the Syrian opposition and persuade them to come together and persuade them to present a more united front so that we can work with them."