David Cameron has heaped pressure on Russia to back tough action against the Syrian regime amid fears the country is sliding into civil war.
The Prime Minister also called on Syrian president Bashar Assad to give up power to avert more chaos and bloodshed.
Speaking during a visit to Afghanistan, 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 (Russian) president (Vladimir) 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 Assad's brother-in-law and the defence minister.
Mr Cameron added: "I don't think any regime that carries out acts as they have should survive."
Urging Russia and China to back other UN nations desperate to take action, he added: "I would appeal to those that have held out in the past against tough action against Syria that what more evidence do we need?
"I think it is in everyone's interest that the transition takes place."
The UN is due to vote this afternoon on a new British-crafted Syria resolution that threatens non-military sanctions against Assad's regime if he does not withdraw troops and heavy weapons from populated areas within 10 days.
The text is tied to Chapter 7 of the UN Charter, which could eventually allow the use of force to end the conflict.
A vote was postponed for another frantic bout of diplomacy yesterday at the request of peace envoy Kofi Annan, after a bomb blast in capital Damascus killed defence minister Dawoud Rajha and Assad's brother-in-law General Assef Shawkat.
The 15-member council must decide the future of a UN observer mission in Syria before its mandate expires on Friday, but the five veto-wielding permanent members - Britain, the US, Russia, China and France - are split over whether stronger action is needed.
The observers were deployed in April to monitor a failed ceasefire under Mr Annan's peace plan.
Speaking in 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 this regime. Clearly, Britain doesn't support violence on either side, but if there isn't transition it's quite clear there's going to be civil war. That is the clear fact, I think, that we can all see on the ground.
"The regime has done some truly dreadful things to its own people and I don't think any regime that carries out acts as they have against their own citizens - and continues to do so, by the way - should survive. I think that regime should go.
"So the message to President Assad is 'It is time for transition, it is time for you to go.'
"The message to President Putin, who I discussed this with at the G20 in Mexico - and the message to all those on the UN Security Council - is 'It's time for the UN Security Council to pass clear and tough messages about sanctions, I believe under Chapter 7 of the UN, and to be unambiguous about this.'
"Obviously, we are a UN Security Council with permanent members, and permanent members that have vetoes. We can't pass these things without everybody stepping up to the plate and taking the right action.
"But I would appeal to those who in the past have held against tough action against Syria - what more evidence do we need about a regime that has brutalised its own people?
"The alternative to political transition at the top of Syria is revolution from the bottom in Syria."