Cameron joins in Assad quit calls
Pressure is mounting on Syrian President Bashar al-Assad after the western world was united in calls for him to stand down.
In a co-ordinated move, Britain, the United States, France, Germany and the European Union (EU) all said the time had come for him to step down and end the brutal repression of his own people.
The demands were led by President Barack Obama, who issued a statement accusing Assad of ignoring demands for reform and of "imprisoning, torturing and slaughtering his own people".
"We have consistently said that President Assad must lead a democratic transition or get out of the way," Mr Obama said. "He has not led. For the sake of the Syrian people, the time has come for President Assad to step aside."
He was followed a short time later by a joint statement from Mr Cameron, French President Nicolas Sarkozy and Germany's Chancellor Angela Merkel condemning the regime's "bloody repression of peaceful and courageous demonstrators".
"Our three countries believe that President Assad, who is resorting to brutal military force against his own people and who is responsible for the situation, has lost all legitimacy and can no longer claim to lead the country," they said.
"We call on him to face the reality of the complete rejection of his regime by the Syrian people and to step aside in the best interests of Syria and the unity of its people. Violence in Syria must stop now."
EU foreign policy chief Baroness Ashton also issued a statement, noting "the complete loss of Bashar al-Assad's legitimacy in the eyes of the Syrian people and the necessity for him to step aside".
Mr Obama announced the US was imposing "unprecedented" sanctions against the regime, freezing all Syrian government assets subject to US jurisdiction and banning any transactions involving the regime in Damascus.
Britain, France and Germany said they were "actively supporting" further EU sanctions against the regime.