Airstrikes and artillery killed more than 60 people in the past 24 hours in Aleppo, including dozens at a hospital in a rebel-held neighbourhood, as Syria's largest city was turned once again into a major battleground in the civil war, officials said Thursday.

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

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

Battle-hardened residents were shocked by the bloodshed. 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 neighbourhoods of Sukkari, Kallasa and Bustan al-Qasr.

In one scene, a building's staircase hung sideways and old men were sobbing.

"The walls, cupboards, everything fell on top of them," cried one man. In another, a clearly terrified small girl with pigtails wept silently while held by a man.

A blond girl walked from the rubble behind her mother, questioning why they were bombed. "What have we done?" she cried.

Aleppo's last pediatrician dead

Among the dead from the hospital strike was Dr. Muhammad Waseem Maaz, reportedly the last pediatrician in the city.

"We lost one of the best hearts in this world," Dr. Abdul Aziz, a surgeon in Aleppo and friend of the pedicatrician, told CBC Radio's As It Happens

"We're losing our doctors. We're losing our staffs. The number of doctors inside Aleppo is going down day by day. It's a crisis already. I don't know what the world is waiting for," he told CBC's Carol Off.

Girl pulled alive from rubble of bombed hospital in Syria0:44

In the rebel-held Sukkari neighbourhood, 27 people died as a well-known 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.

UN envoy for Syria Staffan de Mistura appealed to the U.S. and Russia to help revive the peace talks and ceasefire, which he said "hangs by a thread."

However, the violence only escalated.

MIDEAST-CRISIS/SYRIA-ALEPPO

A man removes supplies from the Medecins Sans Frontieres (MSF)-backed al-Quds hospital on Thursday after it was hit by airstrikes, in a rebel-held area of Syria's Aleppo. (Abdalrhman Ismail/Reuters)

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 told The Associated Press, adding it was an attempt by Assad's government to drive residents from Aleppo.

But a citizen journalist said there was little sign of people fleeing the city.

"Where can they go?" said Bahaa al-Halaby.

MIDEAST-CRISIS/SYRIA-ALEPPO

Women walk past damage near the Medecins Sans Frontieres (MSF)-backed al-Quds hospital after it was hit by airstrikes in Aleppo. (Abdalrhman Ismail/Reuters)

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 group, said fighter jets from the international coalition have not carried out any airstrikes in Aleppo in the past 24 hours.

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

AFP_A17SC

Buildings were left destroyed following an air strike on the rebel-held neighbourhood of al-Kalasa in the northern Syrian city of Aleppo. (Ameer Alhalbi/AFP/Getty Images)

With the U.N.-sponsored peace talks in Geneva completely 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 the five-year-old war.

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

Among the 27 dead were 14 patients and staff, 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.

'We're absolutely appalled'

The 34-bed, multistory hospital, the area's main pediatric care centre, was "hit by direct airstrike," according to a statement by Doctors Without Borders, also known by its French acronym MSF.

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, outpatient and inpatient treatment, the MSF 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 MSF and is based in Amman, Jordan.

"We're absolutely appalled," he told AP.

AFP_A17TH

Syrian civil defence volunteers and rescuers remove a baby from under the rubble of a destroyed building following a reported air strike on the rebel-held neighbourhood of al-Kalasa in the northern Syrian city of Aleppo. (Ameer Alhalbi/AFP/Getty Images)

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 amid screams and wailing. Distraught rescue workers tried to keep away onlookers, apparently fearing more bombs.

Shortly after midday Thursday, new airstrikes in rebel-held areas killed at least 20 people in two neighbourhoods, 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.

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

Mideast Syria

In this Sunday, April 24, photo made from video posted online by the Syrian Civil Defense White Helmets, people stand in rubble after airstrikes and shelling hit Aleppo. More airstrikes were carried out in the city overnight on Wednesday. (Syrian Civil Defense via Associated Press)

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

The ICRC 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.

Ceasefire further frayed

The partial ceasefire 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 group 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.

Since April 19, nearly 200 people have died, including 44 in an airstrike on a market in a rebel-held area in northern Syria's Idlib province. Dozens of civilians also were killed by rebel shelling in government-held areas. There also has been shelling in Damascus, along with a car bombing — both rare for the capital.