Rebel fighters are preparing to leave eastern Aleppo under a deal that could end more than four years of fierce fighting in the Syrian city.
Government buses have been brought in, but the evacuation is being delayed.
Russia, a key ally of President Bashar al-Assad, said on Tuesday military action had ended. City residents say there has been no fighting since then.
Russia also said that government troops had re-established control over the last areas held by the rebels.
The rebels had been squeezed into ever smaller areas of the city in recent months in a major government offensive backed by Russian air power.
News of the evacuation deal came on Tuesday as the UN reported summary killings by pro-government forces.
It said it had reliable evidence that in four areas 82 civilians were killed, adding that many more may have died.
The UN and the US said the Syrian government as well as Russia and Iran - another key ally of President Assad - were accountable for any atrocities committed in the city.
Syria's government and Russia said the allegations were untrue.
Speaking at an emergency session of the UN Security Council on Tuesday, the Russian ambassador to the UN, Vitaly Churkin, said: "According to the latest information that we received in the last hour, military actions in eastern Aleppo are over."
He earlier said the deal was reached to allow the rebels leave, saying it would take place within hours.
"The civilians, they can stay, they can go to safe places, they can take advantage of the humanitarian arrangements that are on the ground. Nobody is going to harm the civilians," Mr Churkin said.
But the rebels, while confirming the deal, stressed that civilians would also be included in the exodus.
Syria's state media said on Tuesday the rebels would be evacuated through the Ramouseh crossing in the south, and then to rebel-held areas in Idlib province.
But the planned evacuation did not start at dawn on Wednesday as expected.
The Syrian government is now said to be demanding a simultaneous evacuation of their own injured fighters and civilians in the nearby towns of Fua and Kfraya, which are encircled by rebel forces.
The seizure of the eastern part of the city by government forces and the crushing of the rebel enclave there represents a major propaganda victory for the government of President Bashar al-Assad, which now controls virtually all of the major population centres of the country.
But Aleppo - the most populous city before the civil war and the country's financial centre - is the biggest prize.
Its capture represents a victory not just for Mr Assad but also for his Iranian and Russian backers.
Aleppo itself may not matter much on Moscow's strategic chess-board. But the defeat of the rebel opposition there underscores the extraordinary turn-around in President Assad's fortunes.
Before Russia intervened President Assad was on the ropes, his military power crumbling.
External actors have propped up his government in large part to secure their own strategic aspirations. And these aspirations will play an important part in deciding what comes next.