BEIRUT, Lebanon — Thousands of people were sent fleeing for their lives on Monday as rebel fighters lost a large stretch of territory in the northern Syrian city of Aleppo to government forces and Kurdish fighters, in what could prove to be a turning point in the conflict, both militarily and psychologically.
Residents described desperate scenes of people being killed by shells as they searched for shelter after their homes came under the heaviest bombardment yet in a grinding battle that has destroyed entire neighborhoods of the city, once Syria’s largest and an industrial hub.
At least 4,000 people have been registered with the Syrian Arab Red Crescent in Jibreen, a government-controlled neighborhood in western Aleppo, Jens Laerke, the spokesman for the United Nations office for 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.
That would give the Syrian government control of the country’s five largest cities and most of the more-populous west, leaving the rebels fighting Mr. Assad with only the northern province of Idlib and a few isolated pockets in the provinces of Aleppo and Homs and around the capital, Damascus.
Throughout the day, government troops, backed by allied militias from Iran and the militant group Hezbollah, advanced from the east and north into the rebel-held areas of Aleppo. That included Hanano, one of the first areas to fall, in 2012, and Sakhour.
Kurdish-led militias advanced from the west, from the Kurdish-controlled neighborhood of Sheikh Maksoud, taking the rebel-held district of Sheikh Fares.
Kurdish militias have staked out areas of de facto autonomy in parts of the country but are not entirely aligned with either the government or the rebels. The state news media and opposition activists have portrayed them, however, as working with the government to fight rebels in Aleppo.
If the government takes back the whole city, large parts of Syria would still remain outside government control, as Kurdish groups and the Islamic State militant group hold most of the eastern half of the country. But it could effectively spell the end of the Syrian insurgent movements that sprang up against Mr. Assad after a crackdown on protests in 2011.
“It’s like doomsday,” said Zaher al-Zaher, an antigovernment activist who could communicate only in short bursts of text messages, as internet connections were failing.
Hisham al-Skeif, a member of a local council in the rebel-held eastern districts of Aleppo, said he was scrambling to find housing for families in areas that had been recaptured in the past day.
“The problem today, in this moment, is not water and food,” he said, at one point choking with tears. “We are threatened with slaughtering, slaughtering.”
The advances shattered a standoff that had lasted months, after government forces surrounded and besieged the rebel-controlled parts of the city this year, closing off regular access to food, medicine and other supplies.
The government and its Russian allies had made several offers for civilians and fighters to leave, but there were few takers. People in eastern Aleppo said they did not trust the government to keep them safe, and government officials said the rebels were not allowing them to leave.
But on Monday, there appeared to be little room for compromise.
Government soldiers posted a video of themselves playing the drums in celebration on the outskirts of rebel-held areas, as leaflets were dropped, telling those inside to flee or face death.
“We won’t have any mercy to those who confront the Syrian Army,” one flier read, “but for those who will return to normal life, all the essentials of life will be secured.”
Another leaflet told rebels to abandon hopes that insurgents outside the city would break the siege.
“Don’t be dumb, think about yourselves and your families,” it read. “Victory is coming for the Syrian Arab Army, think quickly because time is passing and it’s not on your side.”
Still, not all of those fleeing chose to head for government-held areas.
Several thousand people fled rebel-held districts on Monday, according to monitoring groups and residents. Some headed for Sheikh Maksoud, where videos showed them scrambling over a berm; others went to government-held areas, where the state news media showed them thanking national leaders.
Others moved south into areas still controlled by rebels, only to find themselves still under bombardment. Modar Sheko, a nurse, fled his house with his brother, who was killed by a shell in the chaos. Their father, too, was killed as he looked for a grave site, several of Mr. Sheiko’s colleagues said.
Mr. Sheiko’s friend Abdelkafi al-Hamdo, a schoolteacher and activist, said in a text message that Mr. Sheiko would now have to choose between burying the dead, accepting condolences and looking for a house for his surviving relatives.
“What should he do?” Mr. Hamdo said.