The United States will break off discussions with Russia over Syria if the bombardment of Aleppo does not stop, US Secretary of State John Kerry has told his Russian counterpart Sergey Lavrov.

Washington's warning to Moscow came in a phone call in which Kerry said he held Russia responsible for using incendiary bombs and so-called "bunker busters", which have reportedly killed whole families as they sheltered in basements.

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Kerry's spokesman, John Kirby, said bilateral agreements over Syria will be under threat "unless Russia takes immediate steps to end the assault on Aleppo".

The warning comes as United Nations Secretary-General Ban Ki-moon compared the city to a "slaughterhouse", as two more hospitals were bombed on Wednesday (28 September).

"Those using ever more destructive weapons know exactly they are doing," said Ki-moon.

"They know they are committing war crimes. Imagine the destruction. People with their limbs blown off, children in terrible pain with no relief {...} Imagine a slaughterhouse. This is worse. Even a slaughterhouse is more humane."

The violence follows the breakdown of the September 2016 ceasefire and the assault on Aleppo by forces loyal to President Bashar al-Assad.

The city's M2 and M10 hospitals - code words given by doctors to obscure their locations according to The Guardian - were hit between 4am and 10am local time - by several strikes from aerial bombing and artillery shells.

M2 hospital medical administrator, Mohamad Zain Khandaka, told Middle East Eye around 20 people were killed and 90 more trapped following the M2 bombing, which came as patients and staff queued for bread at a bakery." The repairs made to the M2 hospital in the last few days were once again completely destroyed," said Khandaka.

According to figures released by Unicef, 96 children have been killed and 223 injured in eastern Aleppo alone since Friday (23 September). Some 100,000 children are among 250,000 residents under siege.

"The children of Aleppo are trapped in a living nightmare," said UNICEF Deputy Executive Director Justin Forsyth. "There are no words left to describe the suffering they are experiencing."