The United States also condemned the attacks on civilian targets, which it said included two hospitals in the Aleppo area - a Medecins Sans Frontieres facility and the Women's and Children's Hospital in Aziz city.

The upsurge in violence comes just days after international powers, meeting in Munich, proposed a 'cessation of hostilities' within a week as a step towards a permanent ceasefire.

Neither the United States nor the United Nations identified who carried out the air strikes, but Russia has been waging an air campaign in and around Aleppo in support of a Syrian government ground offensive.

State Department spokesman John Kirby said the latest development "casts doubt on Russia's willingness and/or ability to help bring to a stop the continued brutality of the Assad regime against its own people."

"That the Assad regime and its supporters would continue these attacks, without cause and without sufficient regard for international obligations to safeguard innocent lives, flies in the face of the unanimous calls by the ISSG, including in Munich, to avoid attacks on civilians," he said.

The surrounding area was strewn with twisted metal, cinderblocks and detritus from the damaged building.

The UN statement indicated the range of civilian targets was far broader, and the death toll much higher, than originally reported from the region.

UN Secretary General Ban Ki-moon was "deeply concerned by reports that missile attacks on at least five medical facilities and two schools in Aleppo and Idlib that killed close to 50 civilians including children and injured many," UN deputy spokesman Farhan Haq said.

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