Syrian rebels are 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 Mr 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 Shiite 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 and bombardment by Mr Assad's forces.

Video footage showed a group of men climbing aboard a green bus, watched by about 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.

Activists said a total of 1,900 people, mainly rebel fighters, were being evacuated, starting with 600 wounded fighters and civilian relatives.

But most of the people boarding the bus in central Homs appeared to be fit men of fighting age.

Later, video showed them arriving in a rebel-held area north of the city. Unlike an evacuation of civilians from Homs in February, activists said they were not detained for checks by security forces and were allowed to keep their light weapons.

The evacuation comes after months of gains by the army, backed by its Lebanese militant ally, Hezbollah, along a strategic corridor of territory linking the capital Damascus with Homs and Mr Assad's Alawite heartland on the Mediterranean.

The final rebel withdrawal from the centre of the city, known as the "capital of the revolution" when protests first erupted against Mr Assad in 2011, would consolidate his military control ahead of the June 3 election.

Mr Assad's opponents have dismissed the ballot as a charade and say no credible election can be held in a country fractured by ongoing civil war, with swathes of territory outside government control, 6 million people displaced and another 2.5 million refugees abroad.

'Capital of the revolution': Homs was the centre of the initial uprising against Mr Assad in 2011. ( Reuters: Yazan Homsy )

Rebels release captives in Shiite towns

Under the deal which allowed the evacuation from Homs, the rebels also agreed to ease their siege of two northern Shiite towns, Nubl and al-Zahraa.

The Syrian Observatory for Human Rights said rebels opened the roads to allow aid into the two towns on Wednesday morning (local time) at the same time as the first buses collected the departing rebel fighters from Homs.

The Homs evacuation is being carefully synchronised with the aid delivery and the release of the captives in Nubl and Zahraa, and the town of Kassab in Latakia province.

One activist said a Russian national and several Iranians were included among those being released by the rebels.

There was no independent confirmation, but Moscow and Tehran have both supported Mr Assad in the three-year civil war.

More than 150,000 people have died in Syria's civil war and fighting regularly kills more than 200 people a day.

Provincial governor Talal Barazi said Wednesday's operation would ultimately clear the whole of Homs city of gunmen and weapons, suggesting rebels would also be evacuated from the suburb of Al-Waer on the city's north-western outskirts.

Rebels in Al-Waer and the central districts around the Old City have held out against Assad's forces after the army drove them out of the ruins of Baba Amr in March 2012 during a ground offensive which followed weeks of shelling.

Since then, the army gradually tightened its grip around the rebel areas, blocking weapons, medical supplies and food.

It allowed hundreds of civilians to leave in February after lengthy UN mediation.

Reuters