The evacuation of opposition-held eastern Aleppo due to start at dawn has been delayed, perhaps until Thursday, with an opposition official blaming Shi'ite militias allied to Assad regime for the hold-up.
A ceasefire agreement brokered by Russia, Assad's most powerful ally, and Turkey ended years of fighting in the city and has given the regime his biggest victory yet after more than five years of war.
Officials in the military alliance fighting in support of Assad could not be reached immediately for comment on why the evacuation was delayed.
But opposition sources said the ceasefire remained in place despite the delay in the evacuation plan.
Sources on Tuesday had given different expected start times for the evacuation. A military official in the pro-Assad alliance had said the evacuation was due to start at 5 a.m. (0300 GMT), while opposition officials had been expecting a first group of wounded people to leave earlier.
However, none had left by dawn, according to a Reuters witness waiting at the agreed point of departure. Twenty buses were waiting there with their engines running but showed no sign of moving into Aleppo's opposition-held eastern districts.
"There is certainly a delay," said Rami Abdulrahman, director of the Syrian Observatory, a war monitor.
Officials with Aleppo-based opponents accused Shi'ite militias backed by Iran of obstructing the Russian-brokered deal. The pro-opposition Orient TV cited its correspondent as saying the plan may be delayed until Thursday.
People in eastern Aleppo have been packing their bags and burning personal possessions as they prepare to leave, fearing looting by the Assad's army and its Iranian-backed militia allies when they restore control.
The evacuation was the culmination of two weeks of rapid advances by the regime and its allies that drove insurgents back into an ever-smaller pocket of the city under intense air strikes and artillery fire.
Dissidents have been supported by the United States, Turkey and Gulf monarchies, but the support they have enjoyed has fallen far short of the direct military backing given to Assad by Russia and Iran.
Russia's decision to deploy its air force to Syria 18 months ago turned the war in Assad's favour after opposition advances across key areas of western Syria.
In addition to Aleppo, he has won back opposition's strongholds near Damascus this year.
However, Assad is far from taking back all of Syria. Wide areas of the country remain in the hands of armed groups including Daesh, which this week managed to retake the desert city of Palmyra from Syria's army.
The United Nations, the United States said the violence in the city, besieged and bombarded for months, represented "modern evil".
The once-flourishing economic centre with its renowned ancient sites has been pulverised during the war, which has killed hundreds of thousands of people, created the world's worst refugee crisis and allowed the rise of Daesh.
As the battle for Aleppo unfolded, global concern has risen over the plight of the 250,000 civilians who were thought to remain in its opposition-held eastern sector before the sudden army advance began at the end of November.
Tens of thousands of them fled to parts of the city held by the regime or by PKK terrorists, and tens of thousands more retreated further into the opposition enclave as it rapidly shrank under the army's lightning advance.
The rout of opposition from their shrinking territory in Aleppo sparked a mass flight of terrified civilians and insurgents in bitter weather, a crisis the United Nations said was a "complete meltdown of humanity". There were food and water shortages in opposition areas, with all hospitals closed.
On Tuesday, the United Nations voiced deep concern about reports it had received of Assad's soldiers and allied Iraqi fighters summarily shooting dead 82 people in recaptured east Aleppo districts. It accused them of "slaughter".
"The reports we had are of people being shot in the street trying to flee and shot in their homes," said Rupert Colville, a U.N. spokesman. "There could be many more."
Fear stalked the city's streets. Some survivors trudged in the rain past dead bodies to the Assad-held west or the few districts still in dissidents' hands. Others stayed in their homes and awaited the regime army's arrival.
For all of them, fear of arrest, conscription or summary execution added to the daily terror of bombardment.
"People are saying the troops have lists of families of fighters and are asking them if they had sons with the terrorists. (They are) then either left or shot and left to die," said Abu Malek al-Shamali in Seif al-Dawla, one of the last opposition-held districts.
Terrible conditions were described by city residents. Abu Malek al-Shamali, a resident in the opposition area, said dead bodies lay in the streets. "There are many corpses in Fardous and Bustan al-Qasr with no one to bury them," he said.