US President Barack Obama said: "The time has come for President Assad to step aside."

It marks a significant increase in pressure on Mr Assad for sending in his army against the protesters.

Meanwhile, UN investigators say the use of violence in Syria "may amount to crimes against humanity".

In a report to the UN Human Rights Council, the investigators said the UN Security Council should refer the issue to the International Criminal Court.

Human rights groups believe about 2,000 people have been killed and thousands arrested since March as Syria's security forces - including tanks, helicopters, gunships and snipers - try to quell dissent that has broken out in much of the country.

President Bashar al-Assad has promised political reforms but has continued to clamp down on the protesters, blaming the unrest on "terrorist groups".