The clearing of the last opposition-held areas of the Syrian city of Aleppo was put on hold on Friday after pro-regime militias demanded that wounded people should also be brought out of two Shi’ite villages being besieged by rebel fighters.

The Syrian Observatory for Human Rights, a Britain-based war monitoring group, said a group of ambulances and cars containing hundreds of civilians and fighters were stopped by pro-regime gunmen at a checkpoint south-west of Aleppo. They later returned to the enclave.

The second day of the operation to take fighters and civilians out of Aleppo’s rebel enclave ground to a halt amid recriminations from all sides after a morning that had seen the pace of the operation pick up.

“Aleppo is now a synonym for hell,” U.N. Secretary-General Ban Ki-moon told reporters. “I very much regret that we had to stop this operation.”

Aleppo had been divided between regime and rebel areas in the nearly six-year civil war, but a lightning advance by the Syrian army and its allies that began in mid-November deprived rebels of most of their territory in a matter of weeks.

Russia said the Syrian army had established control over all districts of eastern Aleppo although regime troops were suppressing isolated areas where rebel fighters continued to resist.

Rebel sources accused pro-regime Shi’ite militias, the so-called Iran-backed Hezbollah, of opening fire on a convoy carrying evacuees from east Aleppo and robbing them. Road blocks went up and a convoy was forced to turn back.

Though both Russia and Iran back regime head Bashar al-Assad, rebels have blamed Tehran and the Shi’ite groups it backs in Syria for obstructing Moscow’s efforts to broker the evacuation of eastern Aleppo.

Rebels in eastern Aleppo went on high alert after pro-regime forces prevented civilians from leaving and deployed heavy weaponry on the road out of the area, a Syrian rebel commander in the city said.

A Syrian official source said the evacuation was halted because rebels had sought to take out people they had abducted with them, and they had also tried to take weapons hidden in bags. This was denied by Aleppo-based rebel groups.

But a media outlet run by the pro-regime Hezbollah group said protesters had blocked the road from the city, demanding that wounded people from the Shi’ite villages of Foua and Kefraya, which are besieged by rebel groups, in nearby Idlib province should also be evacuated.

Iran, one of Syria’s main allies, had demanded that the villages be included in a ceasefire deal under which people are leaving Aleppo, rebel and United Nations officials have said.

The chaos surrounding the Aleppo evacuation reflects the complexity of the war with an array of groups and foreign interests involved on each side.