ALEPPO, Syria/BEIRUT/WASHINGTON: Plans to evacuate besieged rebel districts of Aleppo were under threat 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 President Bashar Assad’s main backers, imposed new conditions on a cease-fire 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 regime forces using suicide car bombs.

Government forces made a new advance against rebels in the Sukkari neighborhood of Aleppo Wednesday night, bringing at least half of the district under the control of the army and its allies, the Syrian Observatory for Human Rights said.

The ferocity of clashes that had raged through much of the day appeared to reduce somewhat in the area because of the advance, the British-based group of activists said.

A cease-fire 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 cease-fire 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 Recep 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.U.S. Secretary of State John Kerry spoke by phone with his Russian, Turkish and Qatari counterparts, stressing the need to continue seeking a steady cease-fire and the resumption of political talks to end the war.

“In all of these conversations, the secretary has stressed the need to continue to try to stop the bloodshed and violence with a meaningful cease-fire,” State Department spokesman John Kirby told a briefing, adding that “whatever was announced yesterday obviously didn’t survive very long due to the regime.”

There was no sign of Iran’s conditions being met. Insurgents fired shells at the two majority-Shiite villages from which Tehran wanted wounded to be evacuated, Al-Foua and Kufreya, in Idlib province west of Aleppo, causing some casualties, the Observatory said.

Russian Foreign Minister Sergey 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 kilometer.

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 airstrikes 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 Shiite 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 Daesh (ISIS), which this week managed to take back the ancient city of Palmyra, once again illustrating the challenge Assad faces re-establishing control over all Syria.