Syrian forces in the northern city of Aleppo pinned down rebel fighters to a sliver of territory on Monday as President Bashar al-Assad’s troops appeared close to capturing their biggest battlefield prize and delivering a crushing loss to the opposition.

Taking full control of Aleppo — whose eastern districts were among the last urban strongholds of the rebels — would not end the more than five-year-long conflict that has ravaged Syria and scattered millions of war refugees across the Middle East and Europe.

But Aleppo’s fall would mark a major setback for rebel factions and leave them — and their Western and Arab allies — struggling for ways to keep the anti-
Assad rebellion alive. 

On social media, some people in the last few rebel-held neighborhoods of Aleppo wondered whether they were posting their final words.

“I am waiting to die or be captured by the Assad regime,” wrote Ameen al-Halabi, a photographer. “Pray for me and always remember us.”

U.N. Secretary General Ban Ki-moon said he was alarmed by reports of atrocities against “a large number” of civilians, including women and children.

Rebel officials said their fighters were pulling out of district after district in the face of overwhelming force by Syrian ground troops backed by Russian air power. By nightfall, rebels controlled no more than a tenth of the eastern Aleppo neighborhoods they had once hoped to use as a seat of power to rival the capital, Damascus.

“The battle of Aleppo has reached its end. It is just a matter of a small period of time, no more, no less . . . it’s a total collapse,” said Rami Abdulrahman, the pseudonym of the director of the Britain-based Syrian Observatory for Human Rights.

He said rebel forces had withdrawn from all districts on the eastern side of the Aleppo River after losing a key area, Sheikh Said, in overnight fighting.

After repeated attempts to take back Aleppo, a pro-government offensive has smashed rebel defenses there and left armed opposition groups ready to surrender. But while rebel officials say they are willing to evacuate the city, pro-Assad forces have continued their seemingly unstoppable march through what remains of the rebel enclave.

Defeat of the rebels in this strategic city, once Syria’s economic powerhouse, would be devastating for the broader fight against Assad’s rule.

It also would give the government an important morale boost a day after the Islamic State militant group’s recapture of the ancient city of Palmyra underscored the Syrian army’s deep manpower shortages.

More than 100,000 people are thought to have fled the fighting in eastern Aleppo over the past two weeks, often arriving in ­government- or Kurdish-held ­areas with nothing. But those who remain have said they cannot leave. 

Some fear getting caught and even disappearing in the dragnet of government detentions. The United Nations has received reports that the rebels have stopped people from leaving.

“They have been killing us for so long, why would they have mercy?” asked one resident, who gave his name as Ahmed al-Shaer.

The Syrian army is in the "final stages" of recapturing Aleppo after a sudden advance that has pushed rebels to the brink of collapse, a Syrian general said. (Reuters)

Inside the last pockets of rebel territory in eastern Aleppo, most residents sheltered in their houses Monday as bombs rained down and government troops came closer. The area has been shattered, and monitoring groups say hundreds have been killed there in the latest offensive.

Residents said Monday that roads in the rebel-held districts were filled with civilians camped out and huddling in the winter cold.

“Life in Aleppo has become a slow death,” Shaer said. “You watch everything and everyone dear to you disappear.”

And on Monday night, the phones of most civilians contacted by The Washington Post appeared to have fallen silent. Their fate remains unknown.

Liz Sly in Beirut, Heba Habib in Stockholm and Zakaria Zakaria in Istanbul contributed to this report.