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 two-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 (ICRC), 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 years-long 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, strikingcivilians 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.

Last fall, Russia intervened with troops and airstrikes to bolster its ally, Syrian President Bashar al-Assad, who has led a campaign of repression.

The United States, too, has carried out airstrikes against Islamic State fighters and al-Qaeda-linked militants in Syria. Iran has provided Assad's government with aid and scores of military advisers, while Turkey has aided Sunni rebels, particularly in Syria's north.

Also on Thursday, the Islamic State said it destroyed three Turkish tanks along the Syrian border, in a sign of the growing conflict between the jihadists and Turkey's government.

But it was the hospital raid late Wednesday that was likely to have the most devastating humanitarian impact, U.N. and other aid officials said.

The al-Quds hospital in Aleppo's al-Sukkari neighborhood had an emergency room, an intensive-care unit and eight doctors and 28 nurses on staff, Doctors Without Borders said. It was "well known locally," the organization said on its Twitter account Thursday.

At least 14 patients and medical staffers — including three doctors — were killed when the hospital was hit by a "direct" strike, the aid group said.

One of the doctors was Mohammed Waseem Maaz, Aleppo's last pediatrician, activists said. Another was a dentist.

The Syrian Observatory for Human Rights, a Britain-based monitoring group, said 27 people were killed in the hospital strike.

The local rescue unit, the White Helmets, said their volunteers were still working to extract bodies from the rubble on Thursday evening.

A separate strike on Thursday killed 20 people, the monitoring group said.

Syrian state television said that rebel shelling had killed 14 people in government-held areas of Aleppo.

Nour Mashhadi, a media activist based in Aleppo, said another airstrike had hit a bakery Thursday morning.

"Military planes are in the sky all the time," Mashhadi said in a telephone interview from Aleppo.

In a statement, the ICRC called the hospital attack "unacceptable" and said it pushed the city closer to the "brink of humanitarian disaster."

"We urge all the parties to spare the civilians," said Marianne Gasser, head of the ICRC mission in Syria. "Don't attack hospitals, don't use weapons that cause widespread damage."

Aleppo has been divided between rebel- and government-controlled sections since 2012.

In recent months, rebels have been encircled by troops and fighters loyal to Assad. Russia has also lent support to the embattled leader, sending troops and aircraft to pound rebel fighters.

In October, U.S. Special Forces strafed a Doctors Without Borders hospital in Kunduz in northern Afghanistan, killing at least 42 patients, medical staffers and caretakers.

President Obama apologized for the airstrike, which the U.S. military described as a mistake during missions to drive out Taliban fighters.

Doctors Without Borders has pressed for a full international investigation and possible war-crimes charges.

The Washington Post's Brian Murphy in Washington and Zakaria Zakaria in Istanbul contributed to this report.