David Cameron heaped pressure on Russian president Vladimir Putin to back UN sanctions against Syria.
The Prime Minister also called on Syrian president Bashar Assad to fall on his sword.
Speaking during a visit to the Afghan capital Kabul, Mr Cameron said: "I have a very clear message for President Assad. It is time for him to go. It is time for transition in the regime. If there isn't transition it's quite clear there's going to be civil war."
He added: "My message to President Putin is it's time for the UN security council to pass clear and tough messages about sanctions."
Mr Cameron was commenting a day after rebels penetrated the heart of Syria's government elite, detonating a bomb inside a crisis meeting in Damascus that killed three leaders of the regime, including President Assad's brother-in-law and the defence minister.
The United Nations is expected to vote later on a new British-crafted Syria resolution after a last-minute delay failed to get key Western nations and Russia to agree on measures to end the violence.
The resolution threatens non-military sanctions against President Assad's government if he does not withdraw troops and heavy weapons from populated areas within 10 days and is tied to Chapter 7 of the UN Charter, which could eventually allow the use of force to end the conflict.