A hospital, blood bank and ambulances are reported to have been hit as Syrian government aircraft and artillery bombarded rebel-held eastern Aleppo.
The director of the Bayan Children’s Hospital said he was sheltering in the facility’s basement, unable to leave.
The UK-based Syrian Observatory for Human Rights said at least 21 people had been killed, five of them children.
The air strikes resumed on Tuesday after a three-week moratorium declared by the government’s ally Russia ended.
Russia also announced the start of a major operation against jihadist militants elsewhere in western Syria that saw the country’s only aircraft carrier used in combat for the first time.
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Aleppo, once Syria’s commercial and industrial hub, has been divided in roughly two since 2012, with the government controlling the west and rebels the east.