Six hospitals in the Syrian city of Aleppo were hit by Government air strikes last week in what a medical relief agency said amounted to war crimes.

Physicians for Human Rights (PHR) said it was the worst week for attacks on medical facilities in the Aleppo region since the beginning of Syria's five-year conflict.

"Since June, we've seen increasing reports of attacks on civilians in Aleppo and strikes on the region's remaining medical infrastructure. Each of these assaults constitutes a war crime," said Widney Brown, PHR's director of programs.

"Destroying hospitals is tantamount to signing thousands of death warrants for people now stranded in eastern Aleppo," he said.

"The bombings, the lack of humanitarian aid, and the failure of the United Nations to deliver any kind of assistance means the death toll may soon be catastrophic."

The warning came as yet another hospital in Aleppo was hit overnight, with the M2 facility run by the Syrian American Medical Society bombed for the third time.

Heavily damaged buildings in the neighbourhood of Bani Zeid on Aleppo's northern outskirts. ( AFP: George Ourfalian )

Meanwhile, Syrian regime forces bolstered by Russian air strikes have recaptured territory in the battleground city of Aleppo, rolling back the short-lived gains of a rebel offensive.

The Syrian Observatory for Human Rights said Syrian Government forces, with the aid of heavy Russian air strikes, seized two hilltops and two small villages in the south-west of Aleppo.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 9 minutes 27 seconds 9 m Doctors under fire in Aleppo as Syrian forces target hospitals ( Sophie McNeill )

Syria Government forces have advanced on Aleppo in recent weeks, imposing a siege on the opposition-held eastern part of the city since early last month.

In a statement, the UN children's fund UNICEF said it is extremely concerned for the safety and wellbeing of children caught up in the violence in Aleppo, reporting that children make up a third of the 300,000 residents trapped in rebel-held besieged neighbourhoods.

The observatory said 25 civilians were killed in Aleppo and Idlib on Wednesday by Syrian and Russian forces including 10 children, while 10 civilians including 4 children were killed overnight in rebel shelling of government-controlled districts in western Aleppo.