Syrian opposition fighters have begun withdrawing from the central city of Homs under a ceasefire deal agreed with government forces, opposition activists say.

Several buses entered the Old City district on Wednesday to withdraw the first of 1,000 fighters surrounded there by government troops, according to the UK-based Syrian Observatory for Human Rights.

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"Three buses have left, carrying 120 people in total, a mixture of wounded and non-wounded civilians and fighters," Abul Hareth al-Khalidi, a rebel negotiator, told the AFP news agency.

The withdrawal of more than 1,000 fighters from Homs is part of a complex deal that also involves easing a rebel siege on two northern Shia Muslim towns loyal to the president, Bashar al-Assad.

The evacuation, which is taking place under United Nations supervision, will mark the end of any rebel presence in the heart of a city once called the "capital of the revolution" against Assad, handing him a major symbolic victory.

A picture posted on the internet showed green buses driving through central Homs towards a collection point for the withdrawing rebel fighters.

Their departure would be staged over several phases, coordinated with the evacuation of residents from Nubl and al-Zahraa in northern Syria, and the entry of humanitarian aid into those two Shia towns.

More than 150,000 people have died in Syria's civil war. Millions more have fled their homes and the government has lost control of swathes of territory across the north and east.

The government has scheduled presidential elections for June 3, a vote likely to give Assad a third term as president.

Assad's opponents have dismissed the election as a farce.

Source: Al Jazeera and agencies