People walk past a burnt bus in the rebel held Seif al-Dawla neighborhood of Aleppo on Thursday.l (Abdalrhman Ismail/Reuters)

Staffan de Mistura, the U.N. envoy for Syria, said Thursday that he would “physically accompany” to safety jihadist fighters who can be persuaded to leave Aleppo to remove what he said is an “easy alibi” being used by Russia and the Syrian government to bomb the city.

With some apparent desperation in his voice, de Mistura said that rebel-occupied eastern Aleppo may face “total destruction” by the end of the year if the fighting is not stopped.

He appealed to what he said were about 900 members of the Front for the Conquest of Syria, the al-Qaeda group formerly known as Jabhat al-Nusra, to stop holding an estimated 250,000 civilians “hostage.”

“If you decide to leave with dignity, and with your weapons, to Idlib or anywhere you wanted to go, I personally am ready physically to accompany you,” de Mistura said.

If the bombing continues, “thousands of Syrian civilians, not terrorists, will be killed and many of them wounded,” he said, adding that “history will judge” the actions of Russia and Syria.

In one drone video, government-held west Aleppo appears verdant and bustling. Another shows rebel-held east Aleppo as a pock-marked wasteland. (Jason Aldag,Ishaan Tharoor/The Washington Post)

Those two countries have continued a relentless bombing campaign over the eastern part of the city, where U.S.-backed opposition forces are mixed with the militants, saying that they are striking only “Nusra terrorists.”

The bombing began within days after a U.S.-Russia brokered cease-fire took effect on Sept. 12. Russia has charged that the United States has not complied with its obligations under the deal to separate opposition fighters from the jihadists. The United States has said that a cessation of all bombing was supposed to be the first step in the agreement, along with the safe and sustained delivery of food and medical supplies to the besieged civilians.

De Mistura said that the total number of opposition fighters in Aleppo is about 8,000, less than a tenth of whom are members of the Front. U.N. humanitarian coordinator Jan Egeland said that at least 376 people have been killed there over the past two weeks, and that the Syrian government had not supplied permits for the safe passage of any humanitarian aid in the country.

Syria canceled the cease-fire a week after it began. Last week, the United States said it was suspending bilateral cooperation with Russia over Syria because it had not stopped the bombing or allowed humanitarian aid to get to Aleppo and other parts of the country.

Russian Foreign Minister Sergei Lavrov said Thursday that he had not seen the details of de Mistura’s suggestion and was not ready to respond. He spoke at a news conference with French Foreign Minister Jean-Marc ­Ayrault, who was in Moscow seeking support for a proposed U.N. Security Council resolution to stop the bombing and allow the distribution of humanitarian assistance. Ayrault said he expected to bring the same plea to Washington on Friday.

Lavrov said Russia had made some proposals on the resolution, but insisted that the separation of forces in Aleppo was a prerequisite to a cease-fire.

Asked about reports in Washington that the Obama administration was considering military intervention to stop Syrian air force bombing, Lavrov said he asked Secretary of State John F. Kerry in a telephone conversation Wednesday, “How true are the reports of certain American media that Washington is considering the options of delivering cruise missile strikes on the airports used by the Syrian air forces?”

Without indicating Kerry’s ­response, Lavrov said that “the Americans already gave hints to [Syrian President Bashar ­al-]Assad by bombing positions of the Syrian army in Deir al-Zour . . . that was a unilateral military action by the U.S.” The bombing, which he said had killed 80 Syrian soldiers, took place in eastern Syria in an area where the Islamic State is active. The Pentagon has said the strike was a “mistake,” has apologized and said it was investigating.

“You can make your own conclusions,” Lavrov said. “I discussed it with Kerry. He told me with absolute confidence this airstrike had been a mistake and we should just turn the page and forget it . . . and not regard it as some kind of tip by the Americans.

“But we are hearing that some people in Washington are in favor of using force in Syria. We hope their opinion will not prevail.”

Lavrov also said that, as the cease-fire began to fall apart, the Americans had proposed that Russian troops defend a humanitarian route into eastern Syria from the north where the government and opposition forces were fighting.

“We said, ‘We don’t want to provide security there on our own . . . let’s do it jointly.’ ” The Americans, Lavrov said, “told us, ‘Oh, no, that’s too dangerous.’ And that was our humanitarian exchange,” he said.

Ayrault said that Lavrov had stated his position on a possible resolution but that “there are some issues that are getting in the way.”

“The situation is extremely grave,” he said. “That’s why I insisted on a face-to-face with Lavrov . . . to look him in the eyes and sincerely say that no one can accept this situation. . . . With these blind bombardments of the civilian population in Aleppo, the Syrian regime is only feeding radicalization. . . . These brutal blind bombings are creating new jihadists and must be stopped.”