The contested city is now one of the main battlegrounds of Syria's devastating civil war, with a cease-fire that has collapsed and peace talks in Geneva stalled.
At least 27 people died as a hospital supported by Doctors Without Borders and the International Committee for the Red Cross and nearby buildings were hit overnight in the rebel-held part of Aleppo.
The UN envoy for Syria appealed today on the US and Russia to help revive the peace talks and a cease-fire, which he said "hangs by a thread."
The chief Syrian opposition negotiator Mohammed Alloush blamed the government of President Bashar Assad for the violence. He told The Associated Press that it shows "the environment is not conducive to any political action."
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About 200 civilians have been killed in the past week, nearly half of them around Aleppo. There has also been shelling in Damascus, along with a car bombing - both rarities for the capital.
With peace talks in Geneva completely deadlocked, Syrians are regarding the escalating bloodshed with dread, fearing that Aleppo is likely to be the focus of the next phase of the war.
Rebel commanders said government forces have been mobilizing soldiers, equipment and ammunition in preparation for a military action in Aleppo.
The well-known al-Quds filed hospital supported by MSF and ICRC and located in the rebel-held district of Sukkari was hit shortly before midnight Wednesday, according to opposition activists and rescue workers. Six hospital staff and three children were among the 27 who died there.
Among those slain was one of the last pediatricians remaining in opposition-held areas of the contested city and a dentist.
The defense agency, also known as the White Helmets, said the hospital and adjacent buildings were struck in four consecutive airstrikes. It said there were still victims buried under the rubble and that the rescue work continued.