Aleppo's health directorate and the World Health Organisation (WHO) say all hospitals in the besieged east of the city are out of service after days of heavy air strikes, though a war monitor said some are still working.

Elizabeth Hoff, the WHO representative in Syria, said a UN-led group of aid agencies based over the border in Turkey "confirmed [on Saturday] that all hospitals in eastern Aleppo are out of service".

However, the Britain-based Syrian Observatory for Human Rights said some hospitals are still operating in the besieged parts of the city, but many residents are frightened to use them because of heavy shelling.

Earlier, Aleppo health director Abdul Baset Ibrahim confirmed the closure of the hospitals and the condemned attacks on them.

In a statement, Dr Ibrahim claimed the attacks were from the Syrian and Russian armies, and intended to prevent people from receiving medical attention.

"We call on all governments, health workers, health organizations to take immediate actions to stop these war crimes and to stand next to their colleagues in Aleppo," he said.

Injured boys at a field hospital after airstrikes on Friday. ( Reuters: Abdalrhman Ismail )

The Observatory said at least 27 people, including children, had been killed in eastern Aleppo on Saturday by dozens of air strikes, barrel bombs and dozens of artillery rounds.

There were intense clashes in the Bustan al-Basha district, it added.

Air strikes have battered the eastern part of Aleppo 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.

Aleppo now focus of civil war

On Friday, UN humanitarian adviser Jan Egeland told the ABC the besieged population of eastern Aleppo faced a "very bleak moment" with no food or medical supplies, winter approaching, and an increasingly fierce attack by Syrian and allied forces.

Sorry, this video has expired UN humanitarian adviser Jan Egeland speaks to The World's Auskar Surbakti

The Syrian Government, backed by the Russian air force and Shiite militias, has this year steadily closed in on eastern Aleppo, first besieging a population estimated by the United Nations to number 270,000 and then launching a major assault in September.

The battle for what was once Syria's largest city is now the central focus of a five-and-a-half-year-old civil war.

US condemnation

In a statement, White House national security adviser Susan Rice said the US condemned "in the strongest terms" the latest air strikes against hospitals, and urged Russia, an ally of Syrian President Bashar al-Assad, to take steps to halt the violence.

"The United States again joins our partners ... in demanding the immediate cessation of these bombardments and calling on Russia to immediately de-escalate violence and facilitate humanitarian aid and access for the Syrian people."

However, the Syrian war could potentially be entering a new phase after the election of Donald Trump as US president.

While Mr Trump's Syrian policy has not been fully spelled out, he has suggested Washington could re-examine its longstanding opposition to Russia's support for Syrian President Bashar al-Assad.

ABC/wires