By Laila Bassam, Lisa Barrington and John Davison

ALEPPO, Syria/BEIRUT (Reuters) – Plans to evacuate besieged rebel districts of Aleppo were under threat on Wednesday as renewed airstrikes and shelling rocked the Syrian city in a bombardment the United Nations said “most likely constitutes war crimes”.

Iran, one of Syrian President Bashar al-Assad’s main backers, imposed new conditions on a ceasefire deal, saying it wanted the simultaneous evacuation of wounded from two villages besieged by rebel fighters, according to rebel and U.N. sources.

There was no sign of that happening. Instead airstrikes, shelling and gunfire erupted in Aleppo and Turkey accused government forces of breaking the truce agreed less than a day before. Syrian state television said rebel shelling killed six people.

There were clashes on the ground later in the day, with rebels saying they launched an attack against government forces using suicide car bombs.

A ceasefire brokered 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. The evacuation of rebel-held areas was expected to start in the early hours of Wednesday, but did not materialize.

The U.N. High Commissioner for Human Rights, Zeid Ra’ad al Hussein, said he was appalled that the deal appeared to have collapsed.

“While the reasons for the breakdown in the ceasefire are disputed, the resumption of extremely heavy bombardment by the Syrian government forces and their allies on an area packed with civilians is almost certainly a violation of international law and most likely constitutes war crimes,” he added.

There was no immediate indication when the evacuation of civilians and rebel fighters might take place but a pro-opposition TV station said it could be delayed until Thursday. Turkish President Tayyip Erdogan and Russian leader Vladimir Putin agreed in a phone call to make a joint effort to start the process, Turkish presidential sources said.

There was no sign of Iran’s conditions being met. Insurgents fired shells at the two majority-Shi’ite villages from which Tehran wanted wounded to be evacuated, Foua and Kefraya, in Idlib province west of Aleppo, causing some casualties, the British-based Syrian Observatory for Human Rights said.

Russian Foreign Minister Sergei Lavrov predicted that rebel resistance in Aleppo would last no more than two or three days. The defense ministry in Moscow said the rebels now controlled an enclave of only 2.5 square km (1 square mile).

Civilians fill containers with water in a rebel-held besieged area of Aleppo, Syria December 14, 2016. REUTERS/Abdalrhman Ismail

RAPID ADVANCES

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

Officials in the military alliance backing Assad could not be reached immediately for comment on why the evacuation had stalled.

U.N. war crimes investigators said the Syrian government bore the main responsibility for preventing any attacks and reprisals in eastern Aleppo and that it must hold to account any troops or allied forces committing violations.

In what appeared to be a separate development from the planned evacuation, the Russian defense 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.

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 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.

FEAR STALKS STREETS

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.

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.

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 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.

“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.

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