Displaced families gather at a makeshift camp in the government-held district of Jibreen in Aleppo, Syria, as civilians have fled the eastern part of the city in the face of an advance by government forces. (George Ourfalian/AFP/Getty Images)

Thousands of Syrian civilians fled fierce fighting in Aleppo on Sunday as government forces moved within a mile of slicing the city’s last rebel-held stronghold in half.

At least 500 people have been killed and more than 1,000 wounded in a 13-day offensive led by President Bashar al-Assad’s troops on the east side of a city that has taken on huge symbolic importance in the Syrian civil war. Under siege and with no food aid left, a quarter million civilians are now trapped there. On Sunday, residents said they had nowhere left to run. 

“My house is full,” said Bassem, who asked to be identified only by his first name for fear of retribution from the government. “The floors are packed but no one is sleeping.”

The fall of east Aleppo would devastate rebel ambitions to hold onto a rump state in northern Syria, and could hasten the government’s recapture of the entire country.

Assad’s soldiers — supported by Russian- and Iran-backed forces — advanced on the rebel-held districts in a pincer movement Sunday, taking the neighborhoods of Jabal Badra and Baadeen in the east and advancing slowly through al-Sakhur in the west.

Their breakthrough had come Saturday with the recapture of Masaken Hanano, the largest rebel-held district of Aleppo and the first to slip from government control. Its fall underscored how far the tide has turned for Assad’s forces, 5½ years into a war that has killed half a million people and displaced most of Syria’s prewar population.

The rebels seemed ascendant when they seized east Aleppo in 2012, boasting that a march on Damascus would be next. Now, they are being bombed and besieged in pockets of land across the country.

A simultaneous advance by Kurdish-led forces Sunday pushed into the Bustan al-Basha neighborhood. The Syrian Observatory for Human Rights, a Britain-based monitoring group, said nearly 1,700 civilians had fled east Aleppo to government-held areas while another 2,500 had left for Sheikh Maksoud. 

“It is the first exodus of this kind from east Aleppo since 2012,” Rami Abdel Rahman, the Observatory’s director, told Agence France-Presse.

On Syrian state television, families were seen waiting for green buses — repurposed from the school run — to leave for government-held areas. Ragged from exhaustion, some carried the young and the elderly on their shoulders. Their destination was unknown.

It was unclear Sunday how long rebel forces could hold out in the final district connecting their territory north to south. “We are in the process of repelling the onslaught and fortifying our defense lines,” said Yasser al-Youssef, a spokesman for the Nour al-Din al-Zinki Brigade, one of the rebel groups.

“All I can see is Assad’s forces advancing. People have been running all day. It’s chaos,” said Ismail Abdullah, a volunteer with the White Helmets rescue group.

Another resident said his entire extended family had fled — tired and fearful — to his house. “My mother-in-law and her three children are sleeping with us now. One of them couldn’t bring his family, his wife refused to leave. They sent their daughters on ahead,” said Wissam Zarqa, an English teacher.

Several residents said they had smelled what they believed to be chlorine gas close to Sunday’s front lines. Government forces are believed to have used the chemical almost daily during the latest offensive, packing it into barrel bombs filled with shrapnel. 

Few treatment options remain for the wounded. East Aleppo’s health system has been decimated by ferocious attacks leaving many to die, undocumented, in family homes and empty buildings.

“Aleppo is too tired to report the number of casualties,” Zarqa said.

This version has been updated to reflect that an estimated quarter million civilians are living in the rebel-controlled part of Aleppo.