Turkish President Tayyip Erdogan and Russian counterpart Vladimir Putin agreed in a telephone call on Wednesday to make a joint effort to start the evacuation of civilians and opposition forces from eastern Aleppo as soon as possible, Turkish presidential sources said.

Putin and Erdogan emphasized the need to prevent the violations of a ceasefire deal that Russia and Turkey brokered on Tuesday. The evacuation stalled early on Wednesday after Damascus’s ally Iran set new conditions, and both regime forces and rebels have since resumed fighting, reported Reuters.

Erdogan told Putin that Turkey was ready to take all possible measures to provide temporary shelter and humanitarian aid following the opening of safe corridors.

Plans to evacuate besieged rebel districts of Aleppo were under threat on Wednesday as renewed air strikes and shelling rocked the city in a bombardment the United Nations said “most likely constitutes war crimes”.

There was no immediate indication when the Aleppo evacuation might take place but a pro-opposition TV station said it could be delayed until Thursday.