BEIRUT, Lebanon — The evacuation of residents from the last rebel-held section in the devastated Syrian city of Aleppo broke down on Friday with thousands of people still trapped inside, as concern escalated about their fate.

The breakdown — a day after Syrian government buses and ambulances had begun taking people out of the besieged area — came as Russia claimed, incorrectly, that all civilians wishing to leave had already been evacuated and that only “irreconcilable” fighters remained, further raising fears among those still trapped.

Tensions came to a head on Friday afternoon, when a convoy of hundreds of evacuees was detained and turned back by pro-government militiamen. They blocked the way, according to rebel and pro-government fighters, because insurgents in Idlib Province, farther north, were blocking an evacuation of civilians from two villages besieged by rebels.