Activists said militants have burned at least five buses assigned to evacuate wounded and sick people from two villages in northern Syria.  (Syrian Arab News Agency via AP)

Evacuation efforts in Syria were on the verge of fresh collapse Sunday, with thousands of people trapped inside the shattered rebel enclave of Aleppo after gunmen burned a rescue convoy bound for government areas in a neighboring province.

The Britain-based Syrian Observatory for Human Rights said evacuations were postponed for an “unknown” period, deepening abject suffering inside eastern Aleppo as the United Nations thrashed out ways to resolve the crisis.

After more than three hours of closed consultations, France and Russia said Sunday that they had agreed on a U.N. Security Council resolution to deploy international monitors to eastern Aleppo to ensure safe evacuations and immediate delivery of humanitarian aid.

Negotiations to revive a complicated exit deal for civilians and fighters in Aleppo have broken down repeatedly since forces loyal to Syrian President Bashar al-Assad forced the rebels’ effective surrender there last week, amid abuses by government and rebel loyalists along the evacuation routes.

And Sunday, hours after the International Committee of the Red Cross said it was hopeful that evacuations could resume, a fresh hurdle emerged: Video footage from the northwestern province of Idlib showed smoke and fire billowing from a convoy of repurposed school buses bound for Fouaa and Kefraya, Shiite villages besieged by rebels that have taken on great symbolic importance for Syrian President Bashar al-Assad and his Iranian backers. Islamist fighters had set fire to the buses, which were sent to evacuate hundreds of wounded people from those government-held villages.

Iran, a Shiite power, had demanded that the area be evacuated as part of the broader deal allowing thousands of people to leave what remains of the rebel-held eastern areas of Aleppo. 

“You pigs are here to help Shiites. You won’t leave alive,” a man is heard saying off camera in one of the video clips. Although the identity of the attackers is not clear, reports suggest that they had the backing of Jabhat Fatah al-Sham, an al-Qaeda-linked Sunni group with significant influence in Idlib.

“Cowardly terrorist attack on civilian buses & killing driver must not end evacuations,” Jan Egeland, a humanitarian adviser to the United Nations, said Sunday in a tweet.

In another video, the militants appeared on camera, chanting that the attack had restored the “dignity of Aleppo’s besieged people.”

But inside the rebel enclave in Aleppo, desperate residents said it only brought more misery, with thousands awaiting evacuation in freezing temperatures as night fell.

The enclave has been reduced to a pinprick of territory after government forces swept in earlier this month, backed by heavy bombing raids. 

The compromise struck Sunday between Russia, an Assad ally that is helping him with troops and weapons, and France offers hope to the beleaguered residents of eastern Aleppo, many of whom are wary of leaving the area unless international observers monitor the evacuations. On Friday, a bus carrying hundreds of evacuees was held up by pro-government militia fighters who were later accused of ordering dozens of people off the bus and then stripping, robbing and reportedly killing several in the group. 

“Tell me, what do we do, what choices do we have?” said Ahmed al-Mashadi, an engineer in eastern Aleppo. “If we leave, we risk death or humiliation in front of the people we love. But if we stay here? That is death, too.”

The United Nations also accused Syrian government troops and a Shiite militia from Iraq last week of carrying out house-to-house executions as they swept through the area.

Samantha Power, the U.S. ambassador to the United Nations, told reporters Sunday that there had been “many, many reports of people being pulled off buses and disappeared, whether into conscription or into torture chambers or killed outright.”

After earlier disagreements, France and Russia said Sunday that they had agreed to the text of a draft Security Council resolution that would allow about 100 U.N. observers to deploy quickly to the area. A vote was expected Monday.

François Delattre, France’s ambassador to the United Nations, said the resolution, if formally approved, could prevent “mass atrocities.” 

The announcement followed a statement last week from Jean-Marc Ayrault, France’s foreign minister, who decried the situation in Syria as “an affront to the human conscience.” 

“There is, now more than ever, an urgent need to put an end to the hostilities in Aleppo,” Ayrault said. “The regime’s supporters, starting with Russia, cannot let this happen and accept a strategy based on revenge and systematic terror without running the risk of becoming accomplices to the crimes.”

When rebels seized eastern Aleppo from the government in 2012, they had hoped to create a seat of power to rival the Syrian capital, Damascus. Four years on, Aleppo’s recapture by government forces has accelerated the demise of the armed opposition, which will now be boxed inside Idlib without any strategic urban centers under its control.

James McCauley in Paris contributed to this report.