At least 27 people have been killed in what has been described as a "catastrophic day" for Aleppo as Syrian government troops continue to bombard rebel held areas of the city.

The only specialised paediatric hospital in the area is now out of service after airstrikes destroyed three floors of the building according to reports from Medicins Sans Frontieres (MSF).

Three other hospitals have also taken direct hits, resulting in casualties among staff and patients and leaving two key surgical hospitals and the largest general hospital out of service.

The World Health Organisation said all the hospitals in the east of the city are expected to remain out of commission, “for several days at least.”

MSF emergency coordinator, Teresa Sancristova said it has been a “dark day for east Aleppo.”

“The severity of the bombing has inflicted huge damage on the few hospitals working around the clock to provide medical care,” she said.

“The attacks have destroyed entire hospitals, electric generators, emergency rooms and wards, forcing them to stop all medical activities.

“It is not only MSF that condemns indiscriminate attacks on civilians or civilian infrastructure, including hospitals, but also humanitarian law.

"The message is simple and I don´t know how to say it any louder: stop bombing hospitals.”

The Syrian government has pledged its new assault would recapture rebel-held areas with "unprecedented bombardment with every type of weapon."

Barrel bombs, mortar rounds and airstrikes fell upon rebel districts through the night according to the Syrian Observatory for Human Rights (SOHR).

"People went to sleep to the sound of bombardment and awoke to the sound of bombardment," said SOHR chief Rami Abdel Rahman. 

Aleppo has been divided between government control in the west and rebel control in the east since 2012.

More than 250,000 people remain in the opposition-held part of the city, which has been besieged by the regime since July.

The intensity of the bombardment has forced residents to stay indoors, leaving streets all but deserted.

The renewed assault ended a period of relative calm in the city after regime ally Russia halted its strikes and organised a series of brief truces intended to convince residents and surrendering rebels to evacuate.

Syria expert Thomas Pierret said regime forces intend to “combine airstrikes with famine resulting from the siege to get rebels to surrender."

"Aleppo is now completely besieged and its residents are starting to die of hunger," he said.

The White House has condemned the airstrikes "in the strongest terms."