Western powers have said the defection of Syrian Prime Minister Riad Hijab is a sign that the regime of President Bashar al-Assad is crumbling.
The White House said the momentum was now with the opposition, while France said the Assad government was "doomed".
Hijab, the most senior Syrian figure to defect, denounced Syria's "terrorist regime" and said he was joining the revolution.
His whereabouts are unknown, although reports say he may head to Qatar.
Clashes have continued in the second city of Aleppo where rebel fighters are resisting a bombardment by government artillery and fighter jets.