US Secretary of State John Kerry on Wednesday threatened to end Washington's engagement with Russia in Syria following a brutal regime offensive against rebels in Aleppo.

"He informed the foreign minister that 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," said US State Department spokesman John Kirby.

Kerry "made clear the United States and its partners hold Russia responsible for this situation, including the use of incendiary and bunker buster bombs in an urban environment, a drastic escalation that puts civilians at risk," Kirby added. 

The US' top diplomat made Washington's intentions clear in a telephone call with his Russian counterpart Sergei Lavrov, according to the State Department.

More than 250 people have died since government forces backed by Russian airstrikes launched a vicious campaign last week to retake rebel-held eastern Aleppo, according to figures from independent war monitors.

Russia ready to talk

Shortly after the US comments, the Russian military said in a statement it was "ready to continue joint work with our American partners on the Syrian issue" and send a delegation to Geneva to "relaunch consultations."

Russia and the United States have exchanged blame for the failure of an early September ceasefire. 

If the ceasefire had gone into effect for more than a week, under the terms of the deal the United States and Russia had agreed to coordinate in targeting the "Islamic State" and al-Qaeda's Syrian branch - something Moscow has proposed for months. 

Moscow accused the rebels of breaking the ceasefire and criticized the United States for failing to get Western-backed rebels to distance themselves from the al Qaeda-linked Levant Conquest Front, one of the most powerful rebel groups on the ground.

The United States blames Russia and Syria for the collapse of the ceasefire, as they started a major offensive to retake Aleppo and bombed a humanitarian convoy - charges Moscow denies.

'Committing war crimes'

UN chief Ban Ki-moon on Wednesday described the onslaught on rebel-held Aleppo as worse than a "slaughterhouse."

"Those using ever more destructive weapons know exactly what they are doing - they know they are committing war crimes," he said. "Deliberate attacks on hospitals are war crimes. Denying people access to essential health care violates international humanitarian law."

Earlier Wednesday, airstrikes and shelling destroyed two out of the eight remaining hospitals in rebel-held Aleppo.

Meanwhile, French Foreign Minister Jean-Marc Ayrault said he would put forward a UN Security Council resolution to establish a ceasefire in Aleppo.

"At this very moment, we are proposing to discuss a resolution to obtain a ceasefire in Aleppo," Ayrault told French lawmakers.

"This resolution will leave everyone facing their responsibilities: Those who don't vote for it, risk being held responsible for complicity in war crimes," the French foreign minister added.

London and Washington last week accused Russia of possibly committing war crimes in Aleppo, allegations which Moscow vehemently denies.

More than 300,000 people have been killed and half the population displaced since the conflict erupted in 2011, when government forces launched a bloody crackdown on peaceful protesters calling for President Bashar al-Assad to stepdown.

ls/kms (Reuters, AP, dpa)