BEIRUT -- A cease-fire to evacuate rebel fighters and civilians from the remaining opposition-held neighborhoods of Aleppo unraveled on Wednesday, once again raising the specter of a bloody end to the battle for Syria’s largest city as residents reported the resumption of shelling and brutal bombing runs.

Opponents of President Bashar Assad accused the government and its allies of scuttling the deal by adding new conditions, including the lifting of a rebel siege on two pro-government Shiite villages in nearby Idlib province.

However, hours after it crumbled, the rebels said the deal was back on.

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Early Thursday, the media arm of Lebanon’s militant Hezbollah group said overnight negotiations had reinforced a cease-fire deal, adding that Syrian rebels will likely begin leaving their last holdout in Aleppo “in the coming hours.”

And a Syrian army official confirmed all was ready for rebels and civilians to start leaving Aleppo “at any moment.”

The Britain-based Syrian Observatory for Human Rights says the fighting stopped in the city around 4 a.m. Thursday.

The buses waiting in Aleppo Wednesday morning were supposed to evacuate fighters and civilians from the last remaining rebel-held section of the city, CBS News correspondent Holly Williams reported. Instead the ceasefire collapsed, there was shelling and more airstrikes, and the buses went away empty.

Several thousand civilians -- men, women and children -- are holed up in around two square miles of territory, with Aleppo now nearly entirely under the Syrian regime’s control.

It’s a major victory for the regime, after four years of fighting in the city, but it was won by indiscriminately killing civilians, with help from Syria’s allies, Iran and Russia.

Hundreds of Turkish and Syrian demonstrators assembled outside the Iranian consulate in Istanbul to blame Tehran for the failed start of a cease-fire deal that aimed to stop the bloodbath in Aleppo.

The demonstrators shouted “Killer Iran, get out of Syria!” and held up banners that read “Save Aleppo” on Wednesday night.

Turkey, opposition activists and rebels say Iran was largely responsible for the initial failure of the evacuation deal as Tehran imposed new conditions in exchange for safe passage out of eastern Aleppo.

Human Rights Watch said the forces battling for control of the Syrian city of Aleppo are plunging its residents into “hell.”

The New York-based group has called in a statement Wednesday on the forces loyal to the Syrian government in particular to halt “unlawful’ attacks on and allow for the safe evacuation of civilians. The group said rebel groups have also shelled government areas, killing and wounding civilians.

Lama Fakih, deputy Middle East director at Human Rights Watch, said civilians in eastern Aleppo “who had a glimmer of hope that the attacks would stop and that aid would finally reach them are instead trapped in a new brutal air and ground attack.”

Earlier Wednesday, Doctors Without Borders said the remaining doctors in the square mile of eastern Aleppo under attack are “terrified” of possible retaliation from pro-government forces and, more than anything, want to be evacuated.

Teresa Sancristoval, the head of the emergency unit for Aleppo with Doctors Without Borders, said the doctors who are regularly in touch with her group “feel abandoned to their fate and with no way out.”

Doctors in the besieged strip have already been overwhelmed, and only one health facility remains operational. Rescuers said bodies and wounded are being left on the streets because of the intensity of the attack.

“People are losing any kind of hope,” she said.