A U.N.-sponsored deal to evacuate more than 2,000 Islamic State fighters and other militants from rebel-held parts of south Damascus has been delayed, a monitor of the war said today, after a rebel commander was killed.

Coaches has been due to give the fighters safe passage to Raqqa, the de facto capital of Islamic State in northern Syria, Lebanese Hezbollah's TV station said.

But the deal fell through after Zahran Alloush, through whose territory the convoy was due to pass, was killed in an air strike on Friday, the broadcaster - Manar - said.

The UK Observatory for Human Rights, an independent monitor that tracks violence across Syria, said in a statement the evacuation had been expected today but was delayed as there was now no secure territory for the militants to pass through.

The TV report said coaches that had arrived on Friday to pick up the fighters and at least 1,500 family members had turned back.

If it went ahead, the evacuation would be a success for the government of President Bashar al-Assad, increasing its chances of reasserting control over a strategic area just 4 km (2.5 miles) south of the centre of the capital.

It also highlights increasing efforts by the U.N. and foreign governments to bring about local ceasefires and safe-passage agreements as steps towards the wider goal of ending Syria's civil war, in which more than 250,000 people have been killed in nearly five years of fighting.

The rebels' capitulation was forced by a government siege over several years that squeezed the flow of food and humanitarian aid, starving many people to death in what rights group Amnesty International has described as a war crime.