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 of 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 empty. 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 mid-morning.

They said aircraft resumed bombing shortly after noon. Some shells fell near and around the one remaining hospital in the one-square mile enclave. It was nearly impossible for the rescuers to account for the dead because of the continued bombings, they said.

“They began to strike as if there’s no such thing as a ‘cease-fire’ or ‘civilian evacuation,’” said media activist Mahmoud Raslan.

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.

Syrian President Bashar Assad, speaking in an interview with Russia Today, said the cease-fire was designed stop his government’s advance in the city and “keep the terrorists and save them.”