Forces loyal to Syrian President Bashar Assad walking past damaged buildings in the government-held Sheikh Saeed district of Aleppo, Syria, during a media tour. Omar Sanadiki/Reuters Syrian rebel officials announced late Tuesday morning that they had reached a cease-fire deal with Russia to evacuate some opposition forces and civilians from the besieged city of eastern Aleppo.

The talks, which were moderated by Turkish officials in Ankara, appeared to culminate in a deal that would allow 40,000 civilians to leave the city, a source within the opposition with knowledge of the talks told Business Insider.

The UN's envoy to Syria, Staffan de Mistura, said in a press conference on Tuesday that the process to evacuate civilians from the city had already begun. Rebels with light arms would be allowed to evacuate the city as well.

"The opposition's armed groups also can leave Aleppo, following the civilian population," a Turkish official tweeted. "They may go to Idlib, according to the agreement."

Russia's envoy to the United Nations said on Tuesday that "the military activities in east Aleppo have stopped" and that the government "has established control over east Aleppo."

The cease-fire deal came amid reports that pro-government forces, including regime troops and Shiite militias backed by Iran — another Assad ally — had summarily executed dozens of civilians, including women and children, as they moved to clear rebel-held areas of Aleppo on Monday.

Many of the civilians executed by pro-government forces were injured and awaiting treatment in the city's makeshift medical facilities, reports from the scene said. Others were trying to flee their homes.

It's "a complete meltdown of humanity," United Nations humanitarian spokesman Jens Laerke said.

People fleeing deeper into the remaining rebel-held areas of Aleppo on Monday. Abdalrhman Ismail/Reuters The government forces also reportedly set fire to a building used to house people who had been displaced in the fighting, with possibly more than 100 children trapped inside.

"According to alarming reports from a doctor in the city, many children, possibly more than 100, unaccompanied or separated from their families, are trapped in a building, under heavy attack in east Aleppo," Unicef said in a statement. "We urge all parties to the conflict to allow the safe and immediate evacuation of all children."

Civilians being treated at the underground Hayat Medical Center in Aleppo, run by the Syrian American Medical Society, were also executed by pro-government forces on Monday, two sources with knowledge of the matter told Business Insider.

Reports initially indicated that the medical center staff had been executed as well, but they managed to escape in time.

Assad's forces were also using forced conscription to take over and clear areas of the besieged half of the city. Images circulated by pro-government sources on Monday showed Syrian men and boys from east Aleppo standing in front of government forces inside a detention camp.

A police brigadier general addressing men who were evacuated from the eastern districts of Aleppo as part of a preparation Sunday to begin their military service. Omar Sanadiki/Reuters

An estimated 100,000 people are still trapped in a "hellish corner" of less than one mile of territory in eastern Aleppo that hasn't yet fallen to government forces, said Rupert Colville, a spokesman for the United Nations' human-rights office. He said in a press conference that the UN had received reports of pro-government forces killing least 82 civilians, including 11 women and 13 children, in four different neighborhoods.

Civilians trying to flee their homes were also being shot in the street by pro-Assad forces, said Laerke, the UN humanitarian spokesman.

"We're seeing the most cruel form of savagery in Aleppo, and the regime and its supporters are responsible for this," Turkey's foreign minister, Mevlut Cavusoglu, said in a press conference from Ankara. "The wounded are not being let out, and people are dying of starvation."

A nurse living in eastern Aleppo told The New York Times that some rebels, too, had threatened those trying to flee the city.

"Why do you want to go to those who are bombing you?" the nurse said one fighter asked her family as they were trying to flee to government-held territory.

"We are dead either way," one civilian replied.

Syrians who have been evacuated from eastern Aleppo reaching out for Russian food aid in the government-controlled Jibreen area in Aleppo on November 30. The text on the bag, which shows the Syrian and Russian national flags, reads in Arabic: "Russia is with you." Omar Sanadiki/Reuters

The Russian Ministry of Defense said on Tuesday that "tales" of hundreds of thousands of civilians trapped in Aleppo were "Russophobic chatter" and that any civilians remaining in the city were being used by rebels as human shields.

The operation to take Aleppo back from the rebels was "successful and humane in every sense in regard to civilians," the Defense Ministry spokesman Maj. Gen. Igor Konashenkov said.

But Mouaz Moustafa, the executive director of the Syrian Emergency Task Force and political director of the nonprofit, pro-opposition organization United for a Free Syria, said that "horrendous war crimes" were being perpetrated against civilians as neighborhoods fell, one by one, to regime troops.

"And it's all being done with Russian aerial cover, including cluster bombs and barrel bombs," he told Business Insider on Monday.

"Aleppo will fall," he added. "I don't understand how it lasted this long. I just fear for the men, women, and children who will be slaughtered in the process."

Moustafa said he had received reports that the regime was also using chemical weapons, such as chlorine and white phosphorus, to clear the rebel-held areas. Those reports are unconfirmed.

"We are people, are being deleted from the human map," Malek, an activist inside Aleppo, told The New York Times. "We have two neighborhoods and one street, and the regime will keep bombing this small area."

Smoke rising as seen from a government-held area of Aleppo on Monday. Omar Sanadik/Reuters The US was shut out of the new round of negotiations hosted in Ankara, two sources close to the opposition have told Business Insider.

The talks were initially aimed at securing a deal to deliver humanitarian aid to eastern Aleppo in exchange for the evacuation of extremists groups like the former Al Qaeda affiliate Jabhat Fateh al-Sham. Separate negotiations between Russia and the US were apparently underway simultaneously to evacuate rebel groups from Aleppo, allowing civilians to stay and receive humanitarian aid.

The rebels proposed instead that civilians be evacuated from the city.

Arizona Senator John McCain and South Carolina Senator Lindsey Graham released a joint statement on Tuesday saying that news of a cease-fire deal in Aleppo was "not a cause to celebrate, but a sure sign of the fate that awaits other Syrian cities."

"Having consolidated its power in Aleppo and paid no price for its war crimes, the Assad regime will use the ceasefire to reset its war machine and prepare to slaughter its way to victory across the rest of the country," the statement read.

"It is heartbreaking that we've reached this point," it continued. "The cold logic of mass graves confronts us yet again, and the name Aleppo will echo through history, like Srebrenica and Rwanda, as a testament to our moral failure and everlasting shame."