Pressure is mounting on the Syrian leader President Assad to step down, after condmenation from the White House and Downing Street.
David Cameron, whilst on his visit to Kabul, has declared: "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."
Cameron also used the opportunity to heap pressure on Russia, adding: "My message to President Putin is it's time for the UN security council to pass clear and tough messages about sanctions."
Meanwhile the White House has stated that President Assad is "losing control", after the incident in Syria on Wednesday when three senior government figures died in a bomb attack.
Mr Assad's brother-in-law, the defence minister and the head of his crisis team were killed in a blast at the national security headquarters.
Rebels admitted the bomb had been planted the day before.
White House spokesman Jay Carney told the press: "I think the incident today makes clear that Assad is losing control.
"All of our partners internationally need to come together to support a transition."