Russia says it will press on with its bombing campaign in Syria, where forces loyal to President Bashar al-Assad are waging a furious assault on Aleppo's rebel-held eastern sector.

Key points: Dozens of civilians killed in recent strikes on Aleppo

Dozens of civilians killed in recent strikes on Aleppo US again calls for humanitarian aid access

US again calls for humanitarian aid access Ban Ki-moon denounces hospital attacks as war crimes

The United States has threatened to suspend its engagement with Russia over the conflict in Syria following escalating attacks on rebel-held parts of Aleppo city, including strikes on two hospitals.

UN chief Ban Ki-moon denounced the attacks — which saw the two largest hospitals in Aleppo's opposition-controlled east hit with air strikes and artillery fire — as "war crimes".

President Bashar al-Assad's forces and his ally Moscow have carried out a barrage of air strikes on eastern Aleppo since Syria's regime announced a bid last week to retake all of the divided city.

Dozens of civilians have been killed, residential buildings have been reduced to rubble and residents of eastern districts — already suffering under a Government siege — are facing severe shortages of food and medical supplies.

The latest bombardment has been some of the worst in Syria's five-year civil war, and comes after the failure of a short-lived ceasefire brokered by Russia and the United States earlier this month.

Moscow and Washington have traded blame over the truce's collapse, with stinging US criticism of Russia's participation in the Aleppo offensive.

On Wednesday, US Secretary of State John Kerry warned Russian counterpart Sergei Lavrov that Washington would end talks on the Syrian conflict unless Moscow halted the assault on Aleppo.

Mr Kerry said the burden was on Russia to stop the assault and ensure humanitarian aid access, his spokesman John Kirby said.

"The United States is making preparations to suspend US-Russia bilateral engagement on Syria ... unless Russia takes immediate steps to end the assault on Aleppo and restore the cessation of hostilities," Mr Kirby said.

'Imagine a slaughterhouse. This is worse'

White Helmets volunteers have won praise for digging people out of bombed buildings. ( AFP: Karam Al-Masri )

Wednesday's attack saw the M10 and M2 hospitals hit before dawn, forcing both to shut temporarily, said Adham Sahloul of the Syrian American Medical Society, which supports both hospitals.

It was unclear who had carried out the bombings, which UN chief Mr Ban denounced before the Security Council.

"Those using ever more destructive weapons know exactly what they are doing. They know they are committing war crimes," he said.

"Imagine the destruction. People with limbs blown off. Children in terrible pain with no relief.

"Imagine a slaughterhouse. This is worse."

AFP