Syria's prime minister has defected from Assad's beleaguered government to "join the revolution" after less than two months in the job.

Riad Hijab, a former agriculture minister, was among four senior politicians to leave cabinet on Monday in a fresh blow to besieged President Bashar al-Assad and his ruling Ba'ath Party.

State television said he had fled to neighbouring Jordan.

"I announce today my defection from the killing and terrorist regime and I announce that I have joined the ranks of the freedom and dignity revolution. I announce that I am from today a soldier in this blessed revolution," Hijab said in a statement read in his name by a spokesman.

Hijab was appointed as prime minister on June 23 following Assad's controversial election win in May.

Omar Ghalawanji, Hijab's deputy prime minister, was named as a temporary replacement, according to reports.

The announcement came hours after a bomb attack ripped through the third floor of the state TV building in Damascus, causing heavy material damage and light injuries.

A deadly uprising has convulsed Syria for the past 17 months.