(CNN) An airstrike on a pediatric hospital in Syria has killed 50 people, rights and humanitarian groups say, as the United Nations warns that the situation in Aleppo has become "catastrophic" amid intensified fighting in recent days.

The U.N. special envoy to Syria, Staffan de Mistura, told reporters in Geneva on Thursday that Syria's nationwide "cessation of hostilities" is under serious threat of collapse, and that a humanitarian disaster is unfolding as violence increases in Aleppo and three other locations.

Al Quds field hospital, situated in a rebel-held neighborhood in Aleppo, was hit by a missile from a fighter jet Wednesday, witnesses said.

The airstrike killed at least 50 people, according to Pablo Marco, operations manager for Doctors Without Borders in the Middle East.

Marco told CNN that at least six of the dead were hospital staff: Two doctors, two nurses, one guard and one maintenance worker. The death toll could still rise.

U.S. Secretary of State John Kerry condemned the attack and pointed a finger of blame at the Syrian government.

"We are outraged by yesterday's airstrikes in Aleppo on the al Quds hospital supported by both Doctors Without Borders and the International Committee of the Red Cross, which killed dozens of people, including children, patients and medical personnel," he said in a statement.

"It appears to have been a deliberate strike on a known medical facility and follows the Assad regime's appalling record of striking such facilities and first responders. These strikes have killed hundreds of innocent Syrians."

Syrian, Russian, American denials

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Syria's state-run SANA news agency ran a statement denying that government planes were responsible for the assault, and Russia's Defense Ministry issued a statement saying it had not carried out the strike. Syrian government forces, backed by Russian air power, have been pursuing a major offensive on Aleppo in recent months

A U.S. military official told CNN that U.S. forces were not operating in the area of the hospital, and their closest strike had been 20 kilometers (12.4 miles) to the north.

Marco, with Doctors Without Borders, told CNN that two barrel bombs hit buildings near the hospital. The injured were rushed to the hospital and relatives hurried there. A third barrel bomb landed at the facility's gate, causing many of the casualties, he said.

"We cannot be certain who is responsible for this attack," he said. "What we know is it is the Syrian government that has been usually using these barrel bombs in the past."

The Syrian Observatory for Human Rights earlier reported that 27 people were killed at the hospital, including at least three children and three doctors.

The hospital strike came amid an increase in violence around Aleppo, a key city in northern Syria, and other regions, despite a nationwide "cessation of hostilities" technically being in force.

U.N. envoy: Truce 'hangs by a thread'

Death toll from the eight days of fighting in Aleppo has increased to 232. Among the casualties are 36 children and 24 women, according to the Observatory.

De Mistura urged the United States and Russia to help revive peace talks, saying the truce was barely holding and "hangs by a thread."

UN's top envoy to Syria, Staffan de Mistura, is urging Russia and the U.S. to revive peace talks.

"We need that to be urgently revitalized," he said.

"In the last 48 hours, we have had an average of one Syrian killed every 25 minutes, one Syrian wounded every 13 minutes."

No date has been set for the next round of Syrian peace talks.

Jan Egeland, chairman of the U.N. task force on humanitarian access in Syria, said there had been a "catastrophic deterioration" in Aleppo and parts of Homs over the past day or two.

He said that if strikes continued, the U.N. would not be able to carry out a planned delivery of aid to 35 besieged locations.

"I can't express how high the stakes are for the next hours and days," he said.

'Brink of humanitarian disaster'

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Al Quds hospital was supported by both Doctors Without Borders, which uses its French acronym, Médecins Sans Frontières, and the International Committee of the Red Cross.

MSF's head of mission for Syria, Muskilda Zancada, said the organization condemned the "outrageous targeting of yet another medical facility" in the country.

"This devastating attack has destroyed a vital hospital in Aleppo, and the main referral center for pediatric care in the area. Where is the outrage among those with the power and obligation to stop this carnage?" she said in a statement.

According to hospital staff, the facility had been reduced to rubble by a direct strike from the air.

The 34-bed hospital, staffed with eight doctors and 28 nurses, had an emergency room, obstetric care, an outpatient department, an inpatient department, an intensive care unit and an operating theater, MSF said in a statement.

The organization had been donating medical supplies to the facility since 2012.

We condemn the destruction of the Al Quds hospital in #Aleppo, depriving people of essential healthcare. Hospitals are #notatarget, #Syria — MSF International (@MSF) April 28, 2016

Among the victims was one of the last few pediatricians in Aleppo, the group's U.S. executive director Jason Cone tweeted.

Marianne Gasser, head of the ICRC mission in Syria, also condemned the attack on the hospital, saying Aleppo was being "pushed further to the brink of humanitarian disaster."

"The recent attack on the ICRC-supported Quds hospital is unacceptable and sadly this is not the first time the lifesaving medical services have been hit," she said.

"We urge all the parties to spare the civilians. Don't attack hospitals, don't use weapons that cause widespread damage."

The ICRC said it had been supporting most of the medical facilities on both sides of Aleppo's front line by providing medicines, equipment and clean water.

Intense fighting

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The same month, President Bashar al Assad's regime had cut off access to the main road from Aleppo to the Turkish border, hampering movement of rebels fighting his government and aid agencies trying to bring supplies to the city.

"This is one of the places that has not really seen any ceasefire," Pawel Krzysiek, the ICRC's spokesman on Syria, told CNN.

However, he said, the organization had seen "a really sharp increase" in the intensity of the violence in Aleppo over the past week.

The carnage made it difficult to deliver aid as scheduled to besieged areas, he said. The last delivery was three weeks ago and stocks were likely about to run out.

"You can imagine the humanitarian situation that has already been very dire, (where they have) not seen electricity for over four years, becomes worse and worse."

Years of conflict

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Syria's conflict started in 2011 and has raged on for years, with more than 250,000 killed, according to the United Nations.

It started as an anti-government protest as Syrians nationwide demanded reform and the release of political protesters.

Government forces fired on the protesters, killing scores. As the government crackdown intensified, the opposition got more organized.