This image made from video and posted online from Validated UGC, shows a building on fire after airstrikes hit Aleppo, Syria, Thursday, April 28, 2016. A Syrian monitoring group and a first-responders team say new airstrikes on the rebel-held part of the contested city of Aleppo have killed over a dozen people and brought down at least one residential building. The new violence on Thursday brings the death toll in the past 24-hours in the deeply divided city to at least 61 killed. (Validated UGC via AP video)
This image made from video and posted online from Validated UGC, shows a building on fire after airstrikes hit Aleppo, Syria, Thursday, April 28, 2016. A Syrian monitoring group and a first-responders team say new airstrikes on the rebel-held part of the contested city of Aleppo have killed over a dozen people and brought down at least one residential building. The new violence on Thursday brings the death toll in the past 24-hours in the deeply divided city to at least 61 killed. (Validated UGC via AP video)

Istanbul — An onslaught of airstrikes in rebel-held areas of the Syrian city of Aleppo has killed scores and destroyed a hospital supported by international aid groups, activists and humanitarian workers said Thursday, prompting the United Nations to warn of a “catastrophic deterioration” that could intensify an already dire humanitarian crisis.

A 2-month-old cease-fire had brought a brief respite to areas of Syria racked by fighting. But in a 24-hour period between Wednesday and Thursday, at least 60 people — including children and doctors — were killed in rebel-controlled neighborhoods of Aleppo, Syria’s largest city and once the country’s commercial center.

Airstrikes Wednesday night collapsed a hospital supported by Doctors Without Borders and the International Committee for the Red Cross, killing patients and staff members, including one of the city’s last pediatricians, the two aid groups said. The strikes were the latest in a series of increasingly violent attacks that have violated the truce, raising fears of a complete breakdown of efforts to end the yearslong conflict.

“I cannot express how high the stakes are for the next hours and days,” the chairman of the United Nations’ humanitarian task force on Syria, Jan Egeland, told reporters in Geneva.

Egeland said Aleppo, which Syrian officials have said will be the target of an upcoming offensive, had suffered “a catastrophic deterioration” in the previous 24 to 48 hours.

It was not immediately clear who carried out the airstrikes.

But the Syrian air force — backed by Russia — has stepped up raids in Aleppo in recent days, striking civilians and rebel factions in what some Syrians fear is a prelude to a wider government offensive on the city. Russia’s foreign ministry on Thursday denied that it had launched any airstrikes in the past few days, despite carrying out air raids in the past.

The chief Syrian opposition negotiator, Mohammed Alloush, blamed the Syrian government for the airstrikes. He told the Associated Press that the latest violence shows “the environment is not conducive to any political action.”

In a statement, U.S. Secretary of State John F. Kerry said he was “outraged” by the airstrikes in Aleppo.

“Russia has an urgent responsibility to press the regime,” Kerry said, “… including in particular to stop attacking civilians, medical facilities, and first responders, and to abide fully by the cessation of hostilities.”

Earlier Thursday, the chief U.N. envoy to Syria, Staffan de Mistura, broke down a stark tally covering recent days: An average of one Syrian has been killed every 25 minutes, he said.

He called on the United States and Russia to intervene to save the cease-fire, brokered by world powers in February, after talks between the government and opposition all but broke down in Geneva.

The opposition walked out of the talks last week to protest alleged cease-fire violations by government forces.

“We must all be ashamed that this is happening on our watch,” U.N. humanitarian chief Stephen O’Brien said in a statement to the Security Council on Thursday.

“You must not squander the opportunity presented by talks in Geneva and by the cessation of hostilities to put an end to the massive human suffering in Syria,” he said.

Syria’s civil war started after government forces brutally suppressed a pro-democracy uprising five years ago. It has morphed into bloody proxy war that has ensnared world powers and given rise to the Islamic State.