GENEVA - The U.N. envoy for Syria called on al-Qaida-linked militants to leave the embattled city of Aleppo in exchange for an end to government and Russian bombardment, warning Thursday that thousands of civilians could be killed and the historic city "destroyed" by year end if conditions do not soon change.

Special envoy Staffan de Mistura urged fighters from Fatah al-Sham Front to leave the city in exchange for peace. The group was previously known as Nusra Front and changed its name after announcing it had split from al-Qaida earlier this year. The U.N. considers it a terrorist organization.

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.

The combined Syrian government and Russian bombardment of the city's rebel-held east has killed 376 people over the last two weeks, the envoy said. While far fewer have been killed in the western side, which has a population of over a million, presumed rebel shelling killed at least eight people on Thursday, Syrian state media and observers said. It marked one of the bloodiest days in recent memory for government-held neighbourhoods of the city.

De Mistura acknowledged that the fighters would "need some guarantees" before an evacuation to another rebel-held part of the country, but said these would have to come from the government. He also called for the local administration in opposition-held eastern neighbourhoods 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 been leading the diplomatic push. Russia, which currently holds the presidency of the U.N. Security Council, called for de Mistura to brief members on Friday morning.

Yet rebel fighters in Aleppo expressed deep skepticism over the terms of de Mistura's proposal. They say the Fatah al-Sham Front has been instrumental to the east's defence, having led an August counter-offensive that briefly broke the government's siege. The U.N. estimates 275,000 people are trapped in eastern Aleppo.

Ammar Sakkar, a military spokesman for Fastiqum rebel group, said the evacuation plan was "a form of trickery" that would allow pro-government forces to carry out a "longer period of killing and crime." He accused the U.N. of holding a "double standard," arguing that before calling for fighters to leave it must "first stop the head of terrorism and stop his own acts of terrorism and crime against the Syrian people," referring to Syrian President Bashar Assad.

"It would have been better if (de Mistura) spoke about protecting civilians and halting the criminal activities that target civilians in Aleppo," echoed Yasser Alyousef, a political spokesman for Nour el-Din el-Zinki insurgent group.

While Assad has not commented on de Mistura's proposals, his remarks during an interview with Denmark's D2 station Thursday indicated he would not be satisfied with the limited rebel evacuation. Insisting his military would retake the whole of Aleppo, the president rejected any distinctions between the array of nationalist to ultraconservative Islamic factions fighting against his authority.

"The moderate opposition is a myth," he told D2. "That's why you cannot separate something that doesn't exist from something that does exists. All of them have the same grassroots."

During the interview, Assad also denied reports by opposition activists and international relief agencies that his government was targeting hospitals and civilian infrastructure.

In his press conference, de Mistura said the presence of 900 Fatah al-Sham Front fighters should not be used as an excuse to besiege and bombard over a quarter of a million people. "Is this going to be the alibi for destroying the city?" he asked.

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

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 neighbourhoods.

"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 neighbohoods, government forces were pushing ahead with their ground offensive. The Observatory and state media said troops advanced in the northern neighbourhood 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 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 aims to fight terrorism.

The Observatory said the blast in Atmeh was a suicide bombing. It said the dead were Turkish-backed opposition fighters. 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 bodies 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.

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Issa reported from Beirut. Associated Press writers Bassem Mroue in Beirut and Suzan Fraser Ankara, Turkey contributed to this report.

News from © The Associated Press, 2016