Less than 24 hours after a ceasefire was agreed, shelling has resumed in eastern Aleppo.
Conflicting reports emerged over who had broken the truce, with violations being reported from both the Syrian forces and the opposition.
The plan to evacuate the rebel-held districts are under threat as Iran imposed new conditions on the deal, stating that a simultaneous evacuation should take place of the wounded by two villages besieged by rebel fighters, according to the UN and rebel sources.
A Beirut-based TV station showed buses, meant to evacuate thousands of civilians from the besieged city, returning to their depots.
There was no sign of when an evacuation might take place, but a pro-opposition TV said it might be delayed until Thursday.
Russian Foreign Minister Sergei Lavrov predicted that rebel resistance would last no more than two or three days, as the defence ministry claimed the rebels now controlled only 2.5 square kilometres.
UN war crimes investigators said the Syrian government was responsible for preventing attacks and reprisals in eastern Aleppo, after numerous reports emerged of violations by pro-regime forces including executions, arbitrary arrests and enforced disappearances.
The investigators also received allegations of rebel groups preventing civilians from leaving the area, and of fighters merging with the civilian population, putting them at risk.