All hospitals in Syria's besieged rebel-held eastern Aleppo are out of service after days of heavy air strikes, its health directorate and the World Health Organization said, though a war monitor said some were still functioning.

Intense air strikes have battered the eastern part of the city since Tuesday, when the Syrian army and its allies resumed operations after a pause lasting weeks. They launched ground attacks against insurgent positions on Friday.

The war monitor, the Britain-based Syrian Observatory for Human Rights, said 48 people, including at least five children, had been killed in eastern Aleppo on Saturday by dozens of air strikes and barrel bombs and dozens of artillery rounds.