Peace talks in Kazakhstan between the Syrian government and rebel factions have concluded with Russia, Turkey and Iran striking a deal on a three-way mechanism to consolidate a ceasefire in the country.

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.

Syrian government forces have launched a counter-offensive under the cover of air strikes in an attempt to regain control of areas they had lost to insurgents the day before in Aleppo, activists and state media said.

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.