This satellite image released by the United Nations shows road...

This satellite image released by the United Nations shows road damage and craters in the Sha'ar district of Aleppo, Syria on Sept. 18, 2016. Credit: AP

The U.N. envoy for Syria called on militant fighters to leave Aleppo and for the Syrian and Russian governments to end their bombing campaign against rebel-held areas of the embattled Syrian city, warning that otherwise thousands of people could be killed by year-end and Aleppo could be "destroyed."

Staffan de Mistura called on an estimated 900 fighters from Fatah al-Sham Front to leave the rebel-held east of the city. The group was previously known as Nusra Front and changed its name after cutting ties with al-Qaida. The U.N. considers it a terrorist organization.

Demonstrating dramatic flair, de Mistura entreated both sides to "look at my eyes" before offering to "personally" escort the fighters to a refuge of their choosing, provided they agree to lay down their arms. He also urged Syrian and Russian forces to halt their aerial bombardment of rebel areas, which has killed hundreds of people in the city in the last two weeks.

De Mistura acknowledged that the Nusra fighters would "need some guarantees" before an evacuation to another rebel-held part of the country, such as Idlib, but said such guarantees would have to come from the government. "I cannot guarantee more than my own personality and body, frankly," he said.

He called for the local administration in opposition-held eastern neighborhoods to remain in place after Fatah al-Sham leaves, with the U.N. establishing a presence there to bring humanitarian supplies to the besieged population.

His proposals marked the first major initiative by the U.N. to help find a way out of the Syria crisis after the United States, citing in part the Aleppo onslaught, suspended its joint effort with Russia to stop the fighting. Those two powers had led the diplomatic push until now.

De Mistura's appeal to Fatal al-Sham Front was all the more poignant because the United Nations doesn't communicate directly with the group because it's on a U.N. terrorism blacklist.

He drew parallels between Aleppo and previous mass killings in Bosnia and Rwanda, insisting that the U.N. would not stop seeking a resolution to Syria's five-and-a-half year war.

"The bottom line is: In a maximum of two months — two and a half months — the city of eastern Aleppo at this rate maybe be totally destroyed ... and thousands of Syrians, not terrorists, will be killed," he said.

Having last month told the U.N. Security Council that half the fighters in Aleppo were Fatah al-Sham militants, de Mistura revised the estimate down to 900 fighters, among a rebel combatant population of 8,000 and a civilian population of a quarter of a million. This backtracking followed a BBC interview last week in which Russian Foreign Minister Sergey Lavrov said 50 percent of the opposition in Aleppo were from Nusra Front, "as confirmed by the United Nations," according to a transcript posted on Russian diplomatic websites.

Activists said the violence in Aleppo eased on Thursday after Syria's military command announced the night before that it planned to scale back bombardment to allow civilians to leave besieged rebel-held neighborhoods.

"There were shellings and air raids, but it was less than in previous days," said activist Bahaa al-Halaby, speaking from Aleppo province near the city.

The Britain-based Syrian Observatory for Human Rights said that although airstrikes have almost stopped on rebel-held neighbohroods, government forces were pushing ahead with their ground offensive. The Observatory and state media said troops advanced in the northern neighborhood of Bustan al-Basha, capturing a sports complex and a nearby housing compound.

Violence continued elsewhere in Syria. An explosion in Atmeh, a northwestern village near the Turkish border, killed at least 29 people including several Turkish-backed opposition fighters, activists said. IS quickly claimed responsibity for the attack via its news agency, Aamaq.

The bombing and the Aleppo standoff underscored the complexity of the Syrian conflict, which pits Syrian President Bashar Assad's forces against rebels trying to oust him, alongside a U.S.-led coalition's fight against Islamic State group. Russia says its year-long air campaign in support of Syrian troops is aimed to fight terrorism.

Assad on Thursday denied reports by opposition activists and international relief agencies that his government is targeting hospitals and civilian infrastructure. He told Denmark's TV2 station that "to say that this is our aim as a government, (that) we give the orders to destroy hospitals or schools or to kill civilians, this is against our interest."

Still, Assad said mistakes are sometimes committed by individuals in any war, and that he could not have remained president if his government had been "committing all these atrocities."

The Observatory, which tracks the conflict through a network of activists on the ground, said the blast in Atmeh was caused by a suicide bomber wearing an explosive belt. It said the dead were Turkish-backed opposition fighters who have been fighting along with Turkish groups against IS since August.

Another group, the Local Coordination Committees, said the blast killed 35 people, including two senior judicial officials from the opposition.

An amateur video posted online shows about 18 dead men lying on the floor outside what appears to be a clinic. Some of the men are in military uniforms. The video appears genuine and corresponds to other AP reporting of events depicted.

Turkish Deputy Prime Minister Numan Kurtulmus described the blast as an "inhuman attack," adding that "those who perpetrated this committed a crime against humanity."

The Atmeh border post is one of several crossings Syrian rebels use to bring in fighters and supplies.

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