In an image taken from video, a rescuer hurries away with a child Thursday after airstrikes devastated parts of Aleppo, Syria. (AP / VALIDATED UGC )

BEIRUT -- A day of airstrikes and artillery fire killed more than 60 people in Aleppo, including more than two dozen at a hospital in a rebel-held neighborhood, turning Syria's largest city once again into a battleground, officials said Thursday.

Aid agencies warn that Aleppo is on the brink of a humanitarian disaster with the collapse of a two-month cease-fire and stalled peace talks.

Staffan de Mistura, the United Nations envoy for Syria, appealed early Thursday to the U.S. and Russia to help revive the peace talks and cease-fire, which he said "hangs by a thread."

With the U.N.-sponsored peace talks in Geneva deadlocked, Syrians are watching the escalating violence with dread, fearing that Aleppo is likely to be the focus of the next, more vicious phase of a war that has lasted more than five years.

While it was not immediately clear who carried out the airstrikes, opposition activists accused the government of carpet-bombing rebel-controlled areas, while Syrian state media said more than 1,000 mortar rounds and rockets were fired at government-held districts, killing 22 people.

Video posted online by opposition activists showed rescuers pulling bodies from shattered buildings in the rebel neighborhoods of Sukkari, Kallasa and Bustan al-Qasr.

The video showed concrete apartment buildings with their facades sheared off in Bustan al-Qasr, where three children were reported killed. One man carried away a boy with the top of his head missing as another man embraced a 12-year-old girl found alive.

Videos from the government-held side showed a street scene of damaged buildings and a motionless boy in an ambulance.

By nightfall, there was no sign that the attacks had stopped.

The intensified violence -- by far the worst since the partial cease-fire began -- coincides with reports of a military buildup outside Aleppo that many fear is a prelude for a government attempt to force a siege of the city's neighborhoods.

Kerry 'Outraged'

In the rebel-held Sukkari neighborhood, a field hospital supported by Doctors Without Borders and the International Committee for the Red Cross was hit overnight, along with nearby buildings, according to opposition activists and rescue workers.

"Those were multiple airstrikes targeting the same area with less than two-minute gaps," said Adnan Hadad, an opposition journalist.

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

Chief opposition negotiator Mohammed Alloush blamed the government of President Bashar Assad for the violence, saying it shows "the environment is not conducive to any political action."

"What is happening is a crime of ethnic and sectarian cleansing by all means," Alloush said, adding that it was an attempt by Assad's government to drive residents from Aleppo.

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

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

A Damascus-based Syrian military official denied the government had hit the hospital. The official spoke on condition of anonymity because he was not authorized to talk to the media.

Russian Defense Ministry spokesman Maj. Gen. Igor Konashenkov also denied bombing any hospitals in Aleppo, saying its aircraft have not flown any missions in the region for several days.

Col. Steve Warren, the spokesman for the U.S.-led campaign against the Islamic State extremist group, said fighter jets from the international coalition have not carried out any airstrikes in Aleppo in the past day.

About 200 civilians have been killed in the past week in Syria, nearly half of them around Aleppo.

The Red Cross called on all parties to stop indiscriminate attacks and to avoid harming civilians or Aleppo would face what it called a new humanitarian disaster.

"Wherever you are, you hear explosions of mortars, shelling and planes flying over," said Valter Gros, who heads the Red Cross' Aleppo office. "Everyone here fears for their lives and nobody knows what is coming next."

Direct Hit on Hospital

The hospital that was hit in Sukkari has been one of the main medical centers for Aleppo since the city became divided in 2012.

Among the 27 dead were 14 patients and staff members, including three children and six employees, officials said. A dentist and one of the last pediatricians in opposition-held areas of Aleppo were among the victims. The toll was expected to rise.

The 34-bed, multistory hospital, the area's main pediatric care center, was "hit by direct airstrike," according to a statement by Doctors Without Borders.

The hospital had an emergency room, an intensive-care unit and an operating room, and its eight doctors and 28 nurses offered services such as obstetric care and outpatient and inpatient treatment, the medical charity said. The group has supported the hospital since 2012.

The 250,000 people still in Aleppo will now have to find an alternative facility for care, said Sam Taylor, who is Syria communications coordinator for Doctors Without Borders and is based in Amman, Jordan.

"We're absolutely appalled," he said.

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

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

Dating to the 1990s, the hospital was renamed for one of the uprising's early victims, Basel Aslan, after the area came under rebel control. Aslan had been detained by security forces and tortured to death, said civil-defense volunteer Ibrahim Alhaj.

The civil defense, also known as the White Helmets, said the hospital and adjacent buildings were struck in four consecutive airstrikes.

Video posted by the White Helmets showed lifeless bodies, including children, being pulled from a building and loaded into ambulances as screams and wailing rang out. Distraught rescue workers tried to keep away onlookers, apparently fearing more bombs.

Casualties from rebel shelling streamed into Al Razi hospital as the wail of ambulance sirens mixed with the thud of explosions in the city streets. Most of the wounded were civilians, including at least three children who were killed, but some were members of the military.

Hassan Anees, the hospital's executive director, said violence had been rising steadily through the week.

Anees said the rebels appeared to have started using more powerful munitions since the cease-fire crumbled in the city more than a week ago.

"First it was mortars, then it was gas-canister bombs, and now it is missiles," he said.

Lifeline in Jeopardy

Shortly after midday Thursday, new airstrikes in rebel-held areas killed at least 20 people in two neighborhoods, the Syrian Civil Defense and the Observatory said.

Videos by activists showed dust and smoke rising from burning buildings as men and women ran from collapsing houses and children cried, looking for their parents.

Syrian lawmaker Omar Ossi, part of the government delegation at the Geneva talks, blamed the escalation on Turkey and Saudi Arabia as patrons of the rebels.

"The Syrian army will be able to regain the initiative and rein in this Turkish interference," he said.

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

Humanitarian officials said the fighting is putting millions at grave risk.

The U.N. won't be able to reach embattled Syrians if the intensified violence continues on and near aid convoys, said Jan Egeland, a special adviser to the U.N. envoy for Syria, adding that in the past three days, one convoy into Homs was hit by a mortar round and another had to stop several times because of air raids.

Egeland decried a "catastrophic deterioration" of the security situation in Aleppo, saying a lifeline for hundreds of thousands of Syrians "may be broken."

"I cannot express how high the stakes are for the next hours and days," Egeland told reporters in Geneva.

The Red Cross said stocks of contingency food and medical aid are expected to run out soon and warned that an escalation in fighting means that they cannot be replenished.

The partial cease-fire that began Feb. 27 held for weeks. Formally called a "cessation of hostilities," it was never meant to be a total truce because it excluded extremists such as the Islamic State and its rival al-Qaida branch, the Nusra Front.

It frayed further recently as casualties mounted, particularly in Aleppo and other parts of northern Syria. Airstrikes this week hit a training center in rural Aleppo for the Syrian Civil Defense, killing five volunteers.

De Mistura, the U.N. envoy, told the Security Council via videoconference that after 60 days, the truce "hangs by a thread."

"I really fear that the erosion of the cessation is unraveling the fragile consensus around a political solution, carefully built over the last year," he said. "Now I see parties reverting to the language of a military solution or military option. We must ensure that they do not see that as a solution or an option."

The talks foundered last week after the main opposition group, the High Negotiating Committee, suspended its formal participation to protest alleged cease-fire violations by the government, a drop in humanitarian-aid deliveries and no progress in winning the release of detainees.

Information for this article was contributed by Sarah El Deeb, Philip Issa, Zeina Karam and Albert Aji of The Associated Press; by Erin Cunningham, Brian Murphy and Zakaria Zakaria of The Washington Post; and by Anne Barnard, Maher Samaan, Hwaida Saad, Nick Cumming-Bruce from Geneva; and Rick Gladstone and staff members of The New York Times.

A Section on 04/29/2016