The evacuation of more than 3,000 Syrians from four areas as part of a population transfer has been postponed a day after a deadly blast killed more than 120 people, many of them government supporters.

A deal to evacuate trapped civilians and fighters in war-ravaged east Aleppo and two Syrian villages has been thrown into doubt after assailants torched six buses assigned to the operation.

America has dismissed an eight-hour pause in attacks by Russian and Syrian forces on rebel-held eastern districts of Aleppo as "too little, too late".

A top commander in Syria's rebranded al Qaida affiliate who was close to terror network leader Ayman al-Zawahri has been killed in an air strike, the group said.

Four staff have been killed in an air strike on a medical facility in Syria, a relief group has said.