Russian bombers drop munitions over Syria. Russian Defense Ministry BEIRUT — A cease-fire deal between rebels and the Syrian government in the city of Aleppo effectively collapsed on Wednesday, with fighter jets resuming deadly air raids over the opposition's densely crowded enclave in the east of the city.

The attacks threatened to scuttle plans to evacuate rebels and tens thousands of civilians out of harm's way, in what would seal the opposition's surrender of the city.

The evacuation was supposed to begin at dawn, but shelling resumed in the morning hours and buses meant to be used in the pullout of rebels and civilians returned to their depots. Activists and fighters trapped in the opposition's last sliver of territory in Aleppo said pro-government forces had struck their district with dozens of rockets since midmorning.

They said aircraft resumed bombing shortly after noon.

"They began to strike as if there's no such thing as a 'cease-fire' or 'civilian evacuation,'" media activist Mahmoud Raslan said. "They've announced they are going to kill us all."

It was not clear whether the planes were Syrian or Russian. The Britain-based Syrian Observatory for Human Rights monitoring group reported at least six people had been killed.

A legal adviser to the rebels accused Iran of foiling the Russia- and Turkey-brokered deal by imposing new conditions on the rebels. Along with Russia, Iran backs President Bashar Assad's government and has committed advisers and elite Revolutionary Guard forces to the government's war. Turkey backs the rebels fighting to topple Assad.

Osama Abo Zaid, the adviser, said Iran was imposing new conditions for the truce, demanding that the remains of Iranians killed in Aleppo be returned and that Iranian hostages held in rebel-controlled Idlib province be released. He said the conditions were "exclusively sectarian and crippling."

Syria's President Bashar al-Assad (R) meets with Iranian Foreign Minister Mohammad Javad Zarif in Damascus, Syria, in this handout photograph released by Syria's national news agency SANA on August 12, 2015. Reuters

The Syrian government, meanwhile, withdrew its green-colored buses from the evacuation point at the edge of the city of Aleppo's opposition enclave. The Lebanese al-Manar TV, the media arm of the Lebanese militant Shiite group Hezbollah fighting alongside Assad's forces, broadcast footage of the buses leaving the evacuation point empty and said government forces had resumed fighting with rebels in the city.

Mohammed Abu Jaafar, the head of forensics in eastern Aleppo, said eastern Aleppo residents felt "duped."

"People have left their shelters .... to be ready for the evacuation. I can't describe it," Abu Jaafar said. "Since the morning, they started to target the areas where people have gathered ... these people were walking to the crossings designated for exit."

Activists in eastern Aleppo blamed the violence on pro-government forces, saying they shot first. Raslan said he was reporting for a Turkish agency when a rocket crashed nearby at about 10:15 a.m. He shared an audio recording of the explosion with the Associated Press. He was unharmed.

Free Syrian Army fighters fire their weapons during what the FSA said were clashes with forces loyal to Syria's President Bashar al-Assad in Aleppo's Karm al-Jabal district October 15, 2013. Thomson Reuters

The Russian Defense Ministry said in a statement that the rebels "resumed the hostilities" at dawn, trying to break through Syrian government positions to the northwest.

Turkey's foreign minister, Mevlut Cavusoglu, accused the Syrian government and its allies of trying to scuttle the deal. "We see now that the regime and other groups are trying to obstruct this (deal)," he said in remarks quoted by the state-run Anadolu Agency. "This includes Russia, Iran, forces supported by Iran, and the regime."

The surrender of Aleppo's remaining opposition-run neighborhoods to government control would be a turning point in Syria's civil war.

The last-minute deal was mediated by Ankara and Moscow as the rebel enclave rapidly dissolved and ceded more and more territory in the face of the brutal advance by Syrian forces, backed by Russia and assisted by Shiite militias from Lebanon, Iraq, and Afghanistan.

Late on Tuesday, the UN envoy for Syria, Staffan de Mistura, called for immediate access to the former rebel enclave to confirm the end of military operations and to oversee the safe departure of tens of thousands of civilians and opposition fighters. De Mistura was at the Security Council, where an emergency meeting for Aleppo was held.

Staffan de Mistura, United Nations Special Envoy for Syria, shows six with his hands as six days of the truce holding, during a news conference after a meeting of the Task Force for Humanitarian Access at the U.N. in Geneva, Switzerland, March 3, 2016. Denis Balibouse/Reuters

Earlier Wednesday, the pan-Arab al-Mayadeen TV broadcast footage of the government buses idling at the agreed-on evacuation point. The TV said the buses were prepared to move 5,000 fighters and their families to Atareb, an opposition-held town in the northwestern Aleppo countryside.

Brita Haj Hassan, a Syrian opposition official living in exile, said from Luxemburg that there were 800 sick and wounded people requiring immediate medical evacuation from eastern Aleppo. He said the UN and others had informed the opposition the evacuation had been delayed until Thursday but there was no comment from the Syrian government, the United Nations, or aid groups on the ground.

The dramatic developments surrounding Aleppo — which would restore the remainder of what was once Syria's largest city to Assad's forces after months of heavy fighting and a crippling siege — followed reports of mass killings by government forces closing in on the final few blocks still held by the rebels.

UN Secretary-General Ban Ki-moon told the emergency meeting late Tuesday that he had received "credible reports" of civilians killed by pro-government forces as they swept into the last rebel areas in Aleppo.

Bashar al-Ja'afari, Syria's UN ambassador, denied any mass killings or revenge attacks but added it was Syria's "constitutional right" to go after "terrorists," a reference to all opposition fighters.

"Aleppo has been liberated from terrorists and those who toyed with terrorism," he said. "Aleppo has returned to the nation."