BEIRUT, Lebanon — Thousands of people were sent fleeing for their lives on Monday as rebel fighters lost a large stretch of territory to government forces in the northern Syrian city of Aleppo, in what could prove to be a turning point in the conflict, both militarily and psychologically.

Residents described desperate scenes of people’s being killed by shells as they searched for shelter after their homes came under the heaviest bombardment yet of the nearly five-year civil war. Years of airstrikes and shelling have destroyed entire neighborhoods of the rebel-held half of the divided city, once Syria’s largest and an industrial hub.

At least 4,000 people have fled from the rebel-held eastern districts to the city’s government-controlled western side and have registered with the Syrian Arab Red Crescent in Jibreen, a neighborhood there, Jens Laerke, the spokesman for the United Nations office of humanitarian affairs, said on Monday.

As the rebels absorbed the harshest blow since they seized more than half the city four years ago, it seemed increasingly likely that President Bashar al-Assad would eventually manage to take back all of Aleppo.