BEIRUT, Lebanon — The siege in Aleppo is almost over.

Advances by Syrian government forces and their allies have squeezed the fighters and civilians remaining in rebel-held parts of the city into a sliver of territory, spokesmen for the government and the opposition forces said on Monday. The last civilians caught in the shrinking antigovernment enclave issued panicked calls for help.

Late Monday, several residents reported via text and voice messages that they were crowded into abandoned apartments and rainy streets, exposed to shelling and afraid they would be killed or arrested if pro-government forces reached them, as antigovernment activists circulated reports of scores of summary killings in retaken areas.

The United Nations secretary general, Ban Ki-moon, said in a statement on Monday that he was alarmed by reports of atrocities against a large number of civilians, including women and children.

It appeared increasingly likely that the government would gain control of the whole of Aleppo, the largest city in Syria, within days, if not hours. Videos from government-held districts showed people celebrating in the streets, waving flags and honking horns.

It also raises questions about how an increasingly confident President Bashar al-Assad will govern a deeply wounded and divided country where war still rages.

Mr. Assad’s recent military successes have depended heavily on help from Russia, Iran and the Iranian-backed Lebanese Shiite militant group Hezbollah, which are reviled by many of Mr. Assad’s Sunni opponents, and viewed with unease even by some of his supporters because of their growing power in the country. Large areas of Aleppo and other cities and towns have been reduced to rubble by the fighting. And rebel forces, Kurdish militias and Islamic State fighters are still operating in large areas of northern and eastern Syria.

That has been vividly clear in recent days, with Islamic State fighters advancing in the desert city of Palmyra, amid its ancient ruins, as the government focused on Aleppo.

For civilians trapped behind the battle lines in Aleppo, though, more urgent questions loomed. Time appeared to be running out to strike a deal to allow them to evacuate.

Rebel and opposition leaders want evacuees to be given an option to go to territory they control. Many of the remaining civilians refuse to go to government-held territory because they are afraid they would be jailed and tortured there. Others say the bombardment in their districts is too intense to allow any escape. There were also reports in state news media of rebel shelling of government-held areas.

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According to the Russian government, Mr. Assad’s main foreign backer, more than 100,000 people have fled from the rebel enclave to government-held areas, and 2,200 rebel fighters have surrendered. At least 13,000 people have fled in the past 24 hours, it said.

But there are still believed to be thousands of civilians trapped in districts that have been bombarded for more than 24 hours, while the army and its militia allies seized most of the southern part of the city.

“An important call in the name of civilians to help them,” Abdelkafi al-Hamdo, a teacher and antigovernment activist, said in a text message to a group of journalists. “Stop bombing. Take them out.”

The situation was so desperate, residents said, that it was unclear whether rebels and civilians there could even hold a meeting to decide whether to surrender and evacuate.

“Believe me, no one rejects the safety evacuation,” one resident, Yasser Abu al-Joud, said in a text message to the journalists’ group. “All of us are waiting, dying now in the last neighborhoods.”

Hisham al-Skeif is a civilian member of the local council that had tried to govern the rebel-held areas of Aleppo, which once included half the city. Mr. Skeif expressed frustration that while international officials were in contact with rebel leaders, no one appeared to be talking directly to the trapped civilians.

“All the political activists and the media, the relief workers, the unarmed, we are about 1,000, including our families,” he said. Though the government has said it would offer amnesty to anyone who surrenders, “if the regime entered, we will be slaughtered,” he said. “Of course everybody is negotiating with those who are armed, but we are not armed.”

“The armed can defend themselves, but we can’t,” he added.

Mr. Hamdo said in a series of text messages that families in Aleppo were “waiting death together.”

“People are running, don’t know where,” he wrote. “People are under the rubble alive, and no one can save them. Some people are injured in the streets, and no one can go to help them cuz the bombs are always on the same place.”

Some rebel leaders have said that the United States, which backs some rebel groups, sent them a proposal that would allow evacuees their choice of destination, under security guarantees from Russia. But trust has eroded, and no agreement had emerged by Monday night.

Syrian and Russian officials say the offensive in Aleppo will continue until rebels surrender or die.

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“The issue of withdrawing militants is the subject of separate agreements,” Sergei Ryabkov, the Russian deputy foreign minister, told the Ria news agency on Monday. “This agreement has not yet been reached, largely because the United States insists on unacceptable terms.”

The sticking point for months has been a dispute between Russia and the United States over what to do about fighters linked with Al Qaeda who are among the rebel groups in eastern Aleppo. The United Nations estimates that about 10 percent of the 8,000 rebel fighters in the city belong to a Qaeda-linked group, the Levant Conquest Front. The rebels and their international backers say the figure is much lower; the government says it is much higher.

Before the government broke through the rebel defenses, the United Nations was proposing that the Levant Conquest Front leave Aleppo in exchange for three main broad concessions from the government side: an end to bombardment; protection and aid for remaining civilians; and some form of local administration for rebel areas.

But the government rejected any form of self-government, saying that it would “reward terrorists,” while the rebels rejected versions of the deal that did not include it. Now, there is no meaningful rebel territory left for the local opposition council to govern.

“We are hearing terrifying S O S calls from the civilians, but we can’t do anything,” said Bassem al-Hajj, a spokesman for Nour al-Din al-Zenki, one of the main rebel groups in Aleppo. “The fighters are defending, but they are retreating under pressure. Unfortunately, the situation is very bad.”

The focus has now shifted to ending the bombardment and protecting the remaining civilians from reprisals, a reasonable fear in a country where, throughout the six-year conflict, people have been tortured and killed in prison for opposing the government.

“Understand this,” Mr. Hamdo, who said his wife and small daughter were cowering in their apartment with him, wrote on Twitter. “I can’t simply surrender and being captive. I am speaking out, and this is a crime. I might then ask death, and not got it.”

Malek, an activist who asked to be identified only by his first name for fear that he would soon find himself in government territory, said he had moved on Monday to a safer place for the 10th time since the offensive began, along with his cats, Rocky and Loz, the Arabic word for almond.

“Why should I lie? I’m not well,” he said in a series of voice messages. “We are people, are being deleted from the human map. We have two neighborhoods and one street, and the regime will keep bombing this small area.”

Bodies were stuck under the rubble, Malek said, and even members of the White Helmets civil defense group could not rescue anyone, because the group’s equipment had been destroyed and their members scattered by the shelling.

Still attempting to find humor, he said that his cat Rocky had lost “his fiancée” along the way. “Now he’s lonely,” Malek said.

Dr. Salem, a dentist who had kept his clinic open until last week, finally moved to one of the last rebel neighborhoods as his own was taken by government forces. He said he walked through streets shrouded in smoke, and littered with the dead and wounded, to a small area where thousands were crowded in a shrinking space. “There will be a massacre if one rocket falls here,” said Dr. Salem, using only his first name.