Shelling resumed in Aleppo on Dec. 14, and evacuations from the besieged Syrian city stalled despite a deal brokered the day before to allow trapped civilians and rebels to leave. (Jenny Starrs/The Washington Post)

A deal for Syrian rebels and civilians to leave their shattered stronghold in eastern Aleppo broke down Wednesday as fresh fighting raged and evacuation buses were forced to turn away.

Brokered by Russia and Turkey, the pact began with a cease-fire that was supposed to be followed by evacuations at dawn, ending one of the most intense and bloody battles of the five-year civil war. 

But by late morning Wednesday, those terms already seemed defunct.

 “The clashes are violent, and bombardment is very heavy,” said Rami Abdulrahman, director of the Britain-based Syrian Observatory for Human Rights. “It seems as though everything is finished.”

After a month-long offensive, government loyalists have surrounded the rebels in a tiny pinprick of territory of the strategic northern city. Hundreds of civilians have been killed. Many have been buried in shallow mass graves.

On Wednesday, large explosions echoed through audio recordings sent from the enclave. Airstrikes were also reported, after the skies cleared from a heavy storm that had brought respite to civilians trapped below. 

And as news broke that the evacuation buses­ were returning to their depots, relief from the night before turned to confusion and despair. 

“We want to leave . . . we don’t want more massacres, let us leave, what is happening,” wrote Yasser Hemeish, a former accountant, in a message to journalists.

A video shared on social media showed dozens of children huddled in a basement orphanage. Contacted by phone, the director, Asmar al-Halabi, paused as the sound of warplanes echoed over the phone. “Can you hear that?” he asked. “The children are downstairs — they are terrified.”

Rebel officials and a U.N. representative contacted by the Agence France-Presse news agency said the withdrawal deal had broken down after Iran — a key backer of Syrian President Bashar al-Assad — imposed new conditions, saying it wanted the simultaneous evacuation of wounded from two villages besieged by opposition fighters.

Despite Russian and Turkish attempts to revive the deal, there were doubts over whether Iran or the Syrian government, whose cooperation will be needed, are ready to allow rebels to leave now that they are surrounded.

Violence on the ground may already have overtaken diplomacy.

In rejecting the Iranian demand, rebels shelled the villages Wednesday in retaliation. According to a local doctor, pro-government forces­ then responded by launching mortar attacks on the beleaguered town of Madaya, where residents have starved to death.

In Aleppo, a delayed deal and surge in fighting would deepen an already desperate crisis inside the remaining opposition-held districts. U.N. war crimes investigators said Wednesday that the Syrian government would be responsible for preventing revenge attacks by its troops or allied forces­ as they swept through what remained of the rebel area.

The Office of the U.N. High Commissioner for Human Rights said Tuesday that it had received reports that 82 people — including women and children — were shot in their homes or as they fled.

Assad’s forces have been bolstered in Aleppo by Iran-backed Shiite militias with a dark track record of torture and summary executions.

Rebel forces in Aleppo said Tuesday that they had launched their own “military action.” Syrian state television said shelling killed six civilians in the government-held Bustan al-Qasr district. Car bombs also were apparently used elsewhere.

With the tide of Syria’s war turned firmly in Assad’s favor, pockets of resistance across the country have been forced — often through starvation sieges — into surrender deals that culminate in local fighters boarding buses­ and heading to the northern province of Idlib. 

The evacuation of Aleppo’s rebel-held east would be the largest of its kind, and the biggest victory to date for Assad in a conflict that has smashed much of the country and left hundreds of thousands of people dead. 

Pressed by reporters about the U.S. response to the bloodshed, White House press secretary Josh Earnest said the Obama administration continues to support humanitarian assistance and seek a negotiated end to the conflict. He rejected suggestions that the United States has a responsibility to do more to halt the violence.

He said the Syrian regime has “crossed all the lines,” bombing and starving its own citizens, in its attempt to consolidate power. “They do cross just about every line that I can think of,” Earnest said. “And frankly, they cross lines I hadn’t previously thought of.”

“What kind of civilized country is going to support those tactics? But that’s what Russia has done,” he added.

Secretary of State John F. Kerry spoke with Turkish Foreign Minister Mevlut Cavusoglu and Russian Foreign Minister Sergei Lavrov on Wednesday about the situation in Aleppo, State Department spokesman John Kirby said.

Asked what new actions the United States could take in an attempt to change the equation in Syria, Kirby said the United States has a “range of options” at its disposal, but he declined to provide specifics.

Ryan reported from Washington. Heba Habib in Stockholm and Zakaria Zakaria in Istanbul contributed to this report.