BEIRUT >> Simultaneous advances by Syrian government and Kurdish-led forces into eastern Aleppo on Sunday set off a tide of displacement inside the divided city, with thousands of residents evacuating their premises, and threatened to cleave the opposition’s enclave.

Rebel defenses collapsed as government forces pushed into the city’s Sakhour neighborhood, coming within one kilometer (0.6 miles) of commanding a corridor in eastern Aleppo for the first time since rebels swept into the city in 2012, according to Syrian state media and the Syrian Observatory for Human Rights monitoring group.

Kurdish-led forces operating autonomously of the rebels and the government meanwhile seized the Bustan al-Basha neighborhood, allowing thousands of civilians to flee the decimated district to the predominantly Kurdish Sheikh Maqsoud, in the city’s north, according to Ahmad Hiso Araj, an official with the Syrian Democratic Forces.

The government’s push, backed by thousands of Shiite militia fighters from Lebanon, Iraq, and Iran, and under the occasional cover of the Russian air force, has laid waste to Aleppo’s eastern neighborhoods.

An estimated quarter-million people are trapped in wretched conditions in the city’s rebel-held eastern districts since the government sealed its siege of the enclave in late August. Food supplies are running perilously low, the U.N. warned Thursday, and a relentless air assault by government forces has damaged or destroyed every hospital in the area.

Residents in east Aleppo said in distressed messages on social media that thousands of people were fleeing to the city’s government-controlled western neighborhoods, away from the government’s merciless assault, or deeper into opposition-held eastern Aleppo.

“The situation in besieged Aleppo (is) very very bad, thousands of eastern residents are moving to the western side of the city,” said Khaled Khatib, a photographer for the Syrian Civil Defense search-and-rescue group, also known as the White Helmets.

“Aleppo is going to die,” he posted on Twitter.

The Britain-based Observatory, which monitors the conflict through a network of local contacts, said around 1,700 civilians had escaped to government-controlled areas and another 2,500 to Kurdish authorities.

More than 250 civilians have been killed in the government’s bombardment of eastern Aleppo over past 13 days, according to the Observatory.

Locals reported thousands more were moving within the eastern neighborhoods, away from the front lines, but staying inside areas of opposition control.

“The conditions are terrifying” said 28-year-old Modar Sakho, a nurse in eastern Aleppo.

Wissam Zarqa, an English teacher in eastern Aleppo and outspoken government opponent, said some families would stay put in the face of advancing government forces.

Syrian state media reported government forces had seized the Jabal Badro neighborhood and entered Sakhour Sunday after it took control of the Masaken Hanano neighborhood Saturday.

Syrian state TV broadcast a video Saturday showing a teary reunion between a soldier and his family after nearly five years apart, according to the report. It said the family had been trapped in Masaken Hanano.

The Lebanese Al-Manar TV channel reported from the neighborhood Sunday morning, showing workers and soldiers clearing debris against a backdrop of bombed-out buildings on both sides of a wide thoroughfare. Al-Manar is operated by Hezbollah, the Lebanese militant group aligned with the Syrian government.

The Kurdish-led Syrian Democratic Forces’ advance into Bustan al-Basha dealt the opposition a further blow.

Rebels and opposition figures have long accused the SDF and its predecessor groups of conspiring with the government to quash a nationwide revolt.

Araj denied there was any coordination between government and Kurdish-led forces.

“We were responding to calls from residents in Bustan al-Basha to secure the neighborhood,” he said. He added the SDF had entered the area handily as rebel militants fled.

Aleppo used to be Syria’s largest city and commerce capital before its neighborhoods were devastated by the country’s more than five-year-long civil war.

The U.N.’s child agency warned Sunday that nearly 500,000 children were now living under siege in Syria, cut off from food and medical aid, mostly in areas under government control. That figure has doubled in less than a year.

Many are now spending their days underground, as hospitals, schools and homes remain vulnerable to aerial bombardment.

“Children are being killed and injured, too afraid to go to school or even play, surviving with little food and hardly any medicine,” said UNICEF Executive Director Anthony Lake. “This is no way to live — and too many are dying.”

Activists also reported Sunday tens of civilian casualties from a presumed government or Russian airstrike on a village outside Aleppo.

The Local Coordination Committees activist network in Syria reported 15 civilians killed in a Russian airstrike on the village of Anjara, controlled by the opposition in the western Aleppo countryside, and tens of others wounded. Activists usually identify planes by their silhouettes and home base.

The Observatory said the strike was accompanied by raids on other opposition-held villages in the Aleppo countryside.

Meanwhile, Anadolu also reported Sunday that the Islamic State group had used chemical weapons against Turkish-backed Syrian opposition fighters in northern Syria, wounding 22. The report cited a statement by the chief of general staff’s office. The report could not be immediately verified independently.

Later Sunday, Turkey’s emergency relief directorate, which investigated the claim, said it found no trace of chemical warfare. The military was not available for further comment.

Elsewhere in Syria, Israeli aircraft struck a machine gun-mounted vehicle inside the country Sunday, killing four Islamic State-affiliated militants on board after they opened fire on a military patrol on the Israeli side of the Golan Heights, according to the Israeli military.

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Associated Press writer Cinar Kiper in Istanbul contributed to this report.