A Syrian rebel official said pro-Damascus Shi'ite militias were obstructing the departure of people from opposition-held districts of eastern Aleppo on Wednesday under a deal that will hand control of the area to the government.
Officials in the military alliance fighting in support of President Bashar al-Assad could not immediately be reached for comment. Assad has been backed by an array of Shi'ite militias from across the region in his campaign for Aleppo.
Sources had on Tuesday given different expected start times for the evacuation. Opposition officials said they had expected a first group of wounded people to leave on Tuesday evening.
A military official in the pro-Assad alliance had said the evacuation was due to start at 5 a.m. (0300 GMT).
"Iranian militias are turning people back," said Zakaria Malahifji, a Turkey-based official with an Aleppo rebel group.
The Syrian Observatory for Human Rights said no fighter or civilian had left eastern Aleppo as of 5 a.m. (0300 GMT).
A Reuters witness said 20 buses were parked with their engines running on a main road out of the city, awaiting the evacuees. Another dozen were counted nearby.
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