BBC-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”.
Mr Hijab, the most senior Syrian figure to defect, on Monday 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.
White House spokesman Jay Carney said such high-level defections signalled that President Assad’s grip on power was “loosening”.
“If he cannot maintain cohesion within his own inner circle, it reflects on his inability to maintain any following among the Syrian people that isn’t brought about at the point of a gun,” he said.
“The momentum is with the opposition and with the Syrian people.”
French Foreign Minister Laurent Fabius said the latest defection was another sign of the regime weakening and losing support.
“France is convinced the Assad regime is doomed,” he said in a statement.
Last month, Syria’s ambassador to Iraq, Nawaf Fares, deserted to the opposition. Brig Gen Manaf Tlas, who was considered close to President Assad, defected in July.About 30 other generals have crossed into Turkey so far and Turkish news agency Anatolia reported on Monday that another general had fled with five high-ranking officers and more than 30 soldiers.
First news of Mr Hijab’s exit from the Syrian government came from Syrian state TV shortly after a bomb went off at its own headquarters, injuring three people.
Just after announcing the explosion, it broadcast that Mr Hijab – appointed only two months before – had been sacked.
Mr Hijab’s spokesman then appeared on Al-Jazeera TV in neighbouring Jordan saying that the prime minister and his family had fled Syria. He said Mr Hijab was in “a safe location”.
It was widely reported that Mr Hijab had crossed the border into Jordan, although Jordanian state TV later denied this.
The BBC’s Jim Muir in Lebanon says Mr Hijab is expected to move on to Qatar.