By Laila Bassam, Tom Perry and Lisa Barrington

ALEPPO, Syria/BEIRUT (Reuters) – The planned evacuation of rebel districts of Aleppo stalled on Wednesday as air strikes and heavy shelling hit the city and Iran was said to have imposed new conditions on the deal.

Iran, one of Syrian President Bashar al-Assad’s main backers in the battle that has all but ended four years of rebel resistance in the city, wanted a simultaneous evacuation of wounded from two villages, Foua and Kefraya, that are besieged by rebel fighters, according to rebel and U.N. sources.

Rebel groups said that was just an excuse to hold up the evacuation from a shrunken insurgent enclave shattered by a powerful government offensive. A pro-opposition TV station said the operation could now be delayed until Thursday.

A ceasefire brokered on Tuesday by Russia, Assad’s most powerful ally, and Turkey was intended to end years of fighting in the city, giving the Syrian leader his biggest victory in more than five years of war.

But air strikes, shelling and gunfire erupted on Wednesday and Turkey accused government forces of breaking the truce. Syrian state television said rebel shelling had killed six people.

Russian Foreign Minister Sergei Lavrov said however that rebel resistance was likely to end in the next two or three days.

Russian President Vladimir Putin and his Turkish counterpart Tayyip Erdogan will discuss Aleppo later on Wednesday, the Kremlin was quoted as saying.

Officials in the military alliance backing Assad could not be reached immediately for comment on why the evacuation, expected to start in the early hours of Wednesday, had stalled.

Nobody had left by dawn under the plan, according to a Reuters witness waiting at the departure point, where 20 buses stood with engines running but showed no sign of moving into rebel districts.

People in eastern Aleppo packed their bags and burned personal belongings, fearing looting by the Syrian army and its Iranian-backed militia allies.

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

A total of 15,000 people, including 4,000 rebel fighters, wanted to leave Aleppo, according to a media unit run by the Syrian government’s ally Hezbollah.

RAPID ADVANCES

The evacuation plan 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 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 Islamic State, 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 center 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 Islamic State.

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 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.

(Reporting by Laila Bassam in Aleppo and Tom Perry, John Davison and Lisa Barrington in Beirut; Writing by Angus McDowall in Beirut; Editing by Peter Millership, Paul Tait and Giles Elgood)