Syrian state media and Russian authorities have accused rebels in the east of preventing civilians from leaving and using them as 'human shields'.
A ceasefire in the Syrian army's Russian-backed assault on rebel-held Aleppo appeared to expire on Sunday with the UN saying it had been unable to evacuate anyone from the ravaged city.
Moscow had extended the unilateral "humanitarian pause" into a third day until 1600 GMT on Sunday, but announced no further renewal of the truce despite a UN request for longer to evacuate wounded civilians.
Neither residents nor rebels in the opposition-held part of the city heeded calls from Syria's army and Moscow to leave, after weeks of devastating bombardment and a three-month government siege.
The pause began on Thursday, and came after Moscow announced a temporary halt to the Syrian army's campaign to recapture the divided city.
The army opened eight corridors for evacuations, but just a handful of people crossed through a single passage, with the others remaining deserted. "Members of popular civil committees from regime districts entered the eastern neighbourhoods to try to evacuate the injured but failed," Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, said on Sunday.
Syrian state media and Russian authorities have accused rebels in the east of preventing civilians from leaving and using them as "human shields".
More than 2,000 civilians have been wounded since the army launched its offensive to drive the rebels out of the eastern districts they have held since 2012. Nearly 500 people have been killed.
The United Nations had hoped to use the ceasefire to evacuate seriously wounded people, and possibly deliver aid. But a UN official said on Sunday that the requisite security guarantees had not been received. "You have various parties to the conflict and those with influence and they all have to be on the same page on this and they are not," said David Swanson, a spokesman for the UN humanitarian office.
The UN had drawn up a four-day plan that would start with two days of medical evacuations to west Aleppo, rebel-held Idlib province, and Turkey, and continue with more evacuations as well as aid deliveries.
No aid has entered Aleppo since July 7 and food rations will run out by the end of the month, UN chief Ban Ki-moon warned on Thursday.
The UN had asked Moscow to consider extending the pause until Monday evening, but there was no word of any extension as the 1600 GMT deadline passed.
Moscow accuses rebels of preventing civilians from leaving.
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