ALEPPO, Syria/BEIRUT: The evacuation of rebel-held eastern Aleppo due to start at dawn has been delayed, perhaps until Thursday, with an opposition official blaming Iran and its Shiite militias allied to President Bashar Assad 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 Syrian leader 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.

Rebel sources said the ceasefire remained in place despite the delay in the evacuation plan.

"What is stopping the agreement presently is Iranian obstinacy. But the deal still stands, the ceasefire stands until now," said a commander with the rebel Nour al-Din al-Zinki group, speaking in a voice message to Reuters from eastern Aleppo.

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 rebel-held eastern districts.

"There is certainly a delay," said Rami Abdulrahman, director of the Syrian Observatory, a war monitor.

Officials with Aleppo-based rebel groups 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 Syrian army and its Iranian-backed militia allies when they restore control.

In what appeared to be a separate development from the planned evacuation, Russian defense ministry said 6,000 civilians and 366 fighters had left rebel-held districts of Aleppo over the past 24 hours.

RAPID ADVANCES

The evacuation was the culmination of two weeks of rapid advances by the Syrian army and its allies that drove insurgents back into an ever-smaller pocket of the city under intense air strikes and artillery fire.

By taking full control of Aleppo, Assad has proved the power of his military coalition, aided by Russia's air force and an array of Shi'ite militias from across the region.

Rebels groups 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 favor after rebel advances across key areas of western Syria. In addition to Aleppo, he has won back insurgent strongholds near Damascus this year.

The government and its allies have focused the bulk of their firepower on fighting rebels in western Syria rather than ISIS, which this week managed to take back the ancient city of Palmyra, once again illustrating the challenge Assad faces reestablishing control over all Syria.

Russia regards the fall of Aleppo as a major victory against terrorists, as it and Assad characterize all the rebel groups, both Islamist and nationalist, fighting to oust him.

But at 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 pulverized during the war, which has killed hundreds of thousands of people, created the world's worst refugee crisis and allowed the rise of ISIS.

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 rebel-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 government or by a Kurdish militia, and tens of thousands more retreated further into the rebel enclave as it rapidly shrank under the army's lightning advance.

The rout of rebels 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 rebel areas, with all hospitals closed.

"SHOT IN THEIR HOMES"

On Tuesday, the United Nations voiced deep concern about reports it had received of Syrian 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."

The Syrian army has denied carrying out killings or torture among those captured, and Russia said on Tuesday rebels had "kept over 100,000 people in east Aleppo as human shields".

Fear stalked the city's streets. Some survivors trudged in the rain past dead bodies to the government-held west or the few districts still in rebel hands. Others stayed in their homes and awaited the Syrian 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 rebel-held districts.

Terrible conditions were described by city residents. Abu Malek al-Shamali, a resident in the rebel 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.