Britain wants to see a transition of power and not a revolution in Syria, Prime Minister David Cameron has said.
He spoke ahead of talks in the US with Barack Obama at which the violent repression of protests in the Middle Eastern state will be high on the agenda.
The Prime Minister and US president will discuss diplomatic and economic measures to increase pressure on the regime of dictator Bashar Assad, but are not thought likely to consider more radical options like arming rebels or air strikes of the kind seen in Libya last year.
And they are also expected to seek alignment on a timetable for the withdrawal of international troops from Afghanistan, after Mr Cameron acknowledged that the public want to see an "endgame" to a military operation which has now dragged on for more than a decade.
No announcements on further drawdowns are expected, but the two leaders will discuss the timing of the handover in 2013 of lead security responsibility throughout Afghanistan to local troops.
The transition date - possibly in the summer of next year - is expected to be announced at Nato's Chicago summit in May and will allow allied troops to step back into support roles ahead of their gradual return home over the course of 2013 and 2014.
Mr Cameron made clear that he wants to see Assad removed from power by a political process which brings in a new government in Damascus, rather than the escalation of violence which he fears will result from an attempt by the dictator to hang on.
He made no effort to hide his frustration at the ongoing bloodshed in the city of Homs, which has been subjected to a relentless assault by Syrian troops as Assad cracks down on centres of dissent. Mr Cameron said: "We're all frustrated by Syria. What's happening in Homs is completely appalling," the Prime Minister said. "I'm endlessly kicking the tyres and asking what else can be done.
"The shortest way of ending the violence is a transition where Assad goes, rather than a revolution from the bottom. Transition at the top rather than revolution at the bottom."
Mr Cameron and Mr Obama are coming under pressure to speed up disengagement from Afghanistan, following the deaths of six British soldiers and the massacre of 16 Afghan civilians by a renegade US staff sergeant over recent days. But they have both publicly stated they will not "rush for the exit door", but will stick to a plan drawn up by Nato at the Lisbon summit in 2010, which envisaged a handover to lead Afghan control during 2013 and the withdrawal of almost all troops by the end of 2014.