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 on Saturday, after a rebel commander was killed.

Militants and their families were to be given safe passage to Raqqa, the de facto capital of Islamic State in northern Syria.

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 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.