Aleppo: The largest hospital in rebel-held east Aleppo was bombed on Saturday for the second time in days as Syrian government forces pressed a Russian-backed offensive to retake the entire city.

Aleppo, once Syria’s vibrant commercial powerhouse, is now at the heart of a major military campaign by President Bashar Al Assad’s fighters and his steadfast ally Moscow.

The offensive, announced on September 22, has seen dozens of civilians killed and residential buildings flattened in the east, where an estimated 250,000 people live under government siege.

As the situation for civilians grows increasingly dire, the biggest hospital in the rebel-held half of the city was hit by two barrel bombs on Saturday, the medical organisation that supports it said.

“Two barrel bombs hit the M10 hospital and there were reports of a cluster bomb as well,” said Adham Sahloul of the Syrian American Medical Society (SAMS).

Sahloul said a small group of patients and doctors “were inside the hospital for basic triage, bandaging, and cleaning services for emergency cases” when the bombardment began and remain trapped there.

SAMS radiologist and hospital administrator Mohammad Abu Rajab made an urgent call for help on Saturday morning from inside M10.

“The hospital is being destroyed! SOS, everyone!” he said in an audio message distributed to journalists.

M10 had already been hit on Wednesday along with the second-largest hospital in the area, known as M2, in what UN chief Ban Ki-moon denounced as “war crimes”.

That bombardment heavily damaged the two facilities and left only six fully-functional hospitals in the city’s east, according to SAMS.

The battle for Aleppo has sparked some of the most brutal violence since the March 2011 beginning of Syria’s conflict, which has killed more than 300,000 people and displaced over half the population.

The Doctors Without Borders (MSF) medical charity has warned that the recent bombardment was provoking a “bloodbath” in the city.

“Bombs are raining from Syria-led coalition planes and the whole of east Aleppo has become a giant kill box,” MSF director of operations Xisco Villalonga said in a statement on Friday.

But diplomatic efforts to put an end to the fighting across the country have all but collapsed as tensions continue to rise between key outside players.

Russia’s Foreign Minister Sergei Lavrov on Friday accused Washington of protecting the former Al Qaida affiliate Fateh Al Sham Front, once known as Al Nusra Front, in its effort to overthrow Assad.

British Foreign Secretary Boris Johnson said Russia was in danger of becoming “a pariah nation” and the attacks in Aleppo were “unquestionably a war crime”.

The World Health Organisation has called Syria the most dangerous place in the world for health workers, and Aleppo in particular has seen much of its medical infrastructure destroyed or heavily damaged.