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 Jabhat Fatah al-Sham 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 Thursday, Syrian state media and observers said. It marked one of the bloodiest days in recent memory for government-held neighborhoods 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 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. On Thursday, the Russian military strongly warned the United States against striking the Syrian army, noting that its air defense weapons in Syria stand ready to fend off any attack. The statement underlined high tensions between Moscow and Washington after the collapse of a U.S.-Russia-brokered Syria truce and the Syrian army's offensive on Aleppo backed by Russian warplanes.

Meanwhile, rebel fighters in Aleppo expressed skepticism over the terms of de Mistura's proposal. They say the Fatah al-Sham has been instrumental to the east's defense. 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.

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, Assad rejected any distinctions between the array of nationalist to ultraconservative Islamic factions fighting against his authority.

In his press conference, de Mistura said the presence of 900 Fatah al-Sham 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.

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.

The Britain-based Syrian Observatory for Human Rights said that although airstrikes have almost stopped on rebel-held neighborhoods, government forces were pushing their ground offensive.

Violence continued elsewhere. An explosion in Atmeh, a northwestern village near the Turkish border, killed at least 29 people including several Turkish-backed opposition fighters, activists said. Islamic State claimed responsibility.