BEIRUT - Syrian rebels said Tuesday an agreement has been reached with Russia for a cease-fire in Aleppo to evacuate remaining civilians and rebels from besieged districts, just as the U.N. released a report claiming civilians were being slaughtered by government and Russian forces converging on the city.

Osama Abu Zayd, a legal adviser for an umbrella group of rebel factions known as the Free Syrian Army, said the cease-fire went into effect Tuesday evening.

He said the first batch will begin evacuating later Tuesday.

Yasser al-Youssef, a spokesman for the Nour el-Din el-Zinki rebel group, confirmed the cease-fire and said the goal is to evacuate civilians and rebels from besieged areas.

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There was no immediate comment from the Syrian government or Russia on the reports.

Pro-government forces reportedly killed 82 civilians “on the spot” as they closed in on the last rebel enclave in Aleppo Tuesday, the U.N. human rights office said earlier Tuesday.

That and other reports of mass killings, which could not be independently confirmed, reinforced fears of atrocities in eastern Aleppo in the final hours of the battle for the city, which has been split between rebel and government control since 2012.

Several residents and opposition activists have told The Associated Press that government forces carried out summary killings of rebels in the streets in neighborhoods captured on Monday, but the Syrian military flatly denied the claims, saying such allegations were “a desperate attempt” to try gain international sympathy.

None of the residents reached by AP witnessed the alleged killings. Their statements reflected the deepening chaos in the remaining rebel-held areas. Mohammed Abu Rajab, the administrator of the last remaining clinic in rebel-held parts of the city, said people who were killed or wounded are being left in the streets.

Aref al-Aref, a nurse and activist in eastern Aleppo, said civilians had taken cover in a medical center in one of the last rebel holdouts after its staff evacuated two days earlier. He said the army killed them, but pressed for details, he said he only heard it from others.

Monther Etaky, a resident of eastern Aleppo, said he also heard reports of summary killings and knows the name of three families who have reportedly been killed.

“The last news about mass executions of the people who are arrested by the Assad regime and reports of hundreds of men disappeared, missing and thousands taken to fight with the Assad army, all that will push people to stay here,” Etaky said.

The U.N.’s children agency said in a statement on Tuesday that there could be more than 100 unaccompanied children trapped in building under fire in eastern Aleppo, citing a report from an unnamed doctor in the city. UNICEF regional director Geert Cappalaere said UNICEF is concerned over the unverified reports of “extrajudicial killings of civilians, including children.”

The U.N.’s human rights office said it has received reports of pro-government forces killing at least 82 civilians in four neighborhoods of the rapidly-shrinking rebel enclave, including 11 women and 13 children.

Spokesman Rupert Colville said the reports recount pro-government forces entering homes and killing some civilians “on the spot” in the former rebel enclave. Speaking to reporters in Geneva, he said the reports came in late the previous evening and that he doesn’t know exactly when the killings took place.

U.N. Secretary-General Ban Ki-moon said in a statement late Monday that he is alarmed over reports of “atrocities” against a large number of civilians, including women and children, in the past hours in Aleppo. While stressing that the United Nations is not able to independently verify these reports, the U.N. chief said he conveyed his grave concern to the relevant parties.

A press release by the U.N. human rights office in Geneva said that multiple sources reported that tens of civilians were shot dead on Monday in al-Ahrar Square in Kallaseh neighborhood, and also in Bustan al-Qasr, by government forces and their allies, including an Iraqi Shiite militia known as al-Nujabaa.

Retaking Aleppo will prove to be Assad’s biggest victory yet in the 5-year-old civil war. Aleppo has long been regarded as a major gateway between Turkey and Syria.

But a government win in Aleppo does not end the conflict - significant parts of Syria are still outside government control and huge swaths of the country are a devastated wasteland. More than a quarter of a million people have been killed since the conflict began in 2011 with peaceful protests against the Assad family’s four-decade rule.