The pullout of Syrian rebels and civilians from their last holdout in the city of Aleppo was delayed on Wednesday, though it wasn't immediately clear what had caused the delay.

The withdrawal was expected to start early in the early morning hours after the rebels the previous day reached a cease-fire deal to evacuate from eastern Aleppo in what is effectively a surrender — and a defining moment in Syria's civil war.

Late on Tuesday, the U.N. envoy for Syria, Staffan de Mistura, called for immediate access to the former rebel enclave to confirm the end of military operations and to oversee the safe departure of tens of thousands of civilians and opposition fighters from the last sliver of eastern Aleppo into which they had been squeezed by advancing Syrian government forces.

De Mistura was at the Security Council where an emergency meeting for Aleppo was held.

The pan-Arab al-Mayadeen TV broadcast on Wednesday footage of Syrian government buses idling at an agreed-on evacuation point. It was not immediately clear what has caused the delay.

The TV said it expected at least another couple of hours of delay. It said the buses are prepared to move 5,000 fighters and their families to Atareb, an opposition-held town in the northwestern Aleppo countryside.

The Syrian government or the opposition in Aleppo have not made any announcements about the delay.