Rebel sources and the Syrian army say the head of one of the most powerful insurgent groups in the eastern Damascus suburbs was among those killed in an air strike on his headquarters.

Zahran Alloush was leader of the Jaysh al Islam group, the largest rebel faction in the area.

The rebels say Russian aircraft fired at least 10 missiles at the group's secret headquarters.

The Syrian army said it had completed aerial strikes targeting what it called terrorist groups.

Jaysh al Islam was one of the groups to attend a recent Saudi-backed meeting in Riyadh and part of the negotiating team due to hold talks with Syria's government in Geneva.

Alloush's death came came as around 2,000 Islamist fighters prepared for evacuation from a rebel-held part of southern Damascus.

They include members of Islamic State and al Qaeda's Nusra Front.

Hezbollah's Manar TV said the fighters would hand over their heavy weapons to the Syrian army.

The evacuation was forced by a government siege lasting several years which has stemmed the flow of food and humanitarian aid to the area.

Their departure, with 1, 500 family members, was brokered by the United Nations.