BEIRUT: Air strikes hit a hospital in a rebel-held area of Syria's Aleppo and killed at least 27 people, including three children and the city's last pediatrician, the Syrian Observatory for Human Rights said on Thursday.
The al-Quds hospital was supported by international medical charity Medecins Sans Frontieres (MSF), which said it was destroyed after being hit by a direct air strike that killed at least three doctors.
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The head of a rescue service put the death toll at 50, saying most of the dead were in a building next door.
Aleppo has been the epicenter of a military escalation that has helped to undermine UN-led peace talks in recent weeks. A cessation of hostilities agreement has unraveled and fighting has resumed on numerous fronts in western Syria.
The city is divided into areas held by the government and rebels.
The Britain-based Observatory said 91 civilians had been killed in air strikes in the past six days in Aleppo and 49 civilians were killed in rebel shelling of government-held areas.
"Destroyed #MSF-supported hospital in Aleppo was well known locally and hit by direct airstrike on Wednesday. Hospitals are #notatarget," an MSF Twitter account said.
Bebars Mishal of the Civil Defence in Aleppo told Reuters that 40 people had been killed in a five-storey building next to the hospital.
A Syrian military source said government warplanes had not been used in areas where airstrikes were reported.
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The Russian defense ministry, which is also conducting air strikes in Syria in support of President Bashar al-Assad, could not immediately be reached for comment. Russia has previously denied hitting civilian targets in Syria.
The Syrian state news agency SANA said nine people had been killed in rebel shelling of residential areas of Aleppo on Thursday.
The Syrian military source said the army has been responding to insurgent attacks in Aleppo, adding: "If the militants continue using this fire and bombardment of civilians, the army will certainly not be quiet about it."
UN envoy Staffan de Mistura said on Thursday the cessation of hostilities agreement was "barely alive".
Peace talks he has convened in Geneva were undermined last week when the main opposition alliance walked out, citing ongoing violence and calling for proper implementation of a UN resolution requiring full humanitarian access to besieged areas.
The talks were making no obvious progress toward discussion of the political transition sought by President Bashar al-Assad's opponents.