Syrian rebels have started withdrawing from the heart of Homs city, leaving an early centre of the revolt against President Bashar al-Assad and handing him a symbolic victory less than a month before his likely re-election.

Two buses carrying the first of many hundreds of fighters left the besieged city centre in an evacuation agreed between insurgents and forces loyal to Assad.

The deal also includes the release of captives held by rebels in Aleppo and Latakia provinces, and the easing of a rebel siege of two Shi'ite towns in northern Syria.

The overwhelmingly Sunni Muslim fighters had held out in the Old City of Homs and neighbouring districts despite being undersupplied, outgunned and subjected to more than a year of siege .

Video footage showed a group of men climbing aboard a green bus, watched by around a dozen men in khaki uniform and black flak jackets marked "police". In front of the bus was a white car with the markings of the United Nations, which helped oversee the operation.