US Secretary of State John Kerry is threatening to cut off contact with Moscow over Syria unless attacks on Aleppo are stopped.
The State Department says Mr Kerry issued the ultimatum in a telephone call to Russian Foreign Minister Sergey Lavrov.
It comes as two hospitals were badly damaged in blasts as government forces intensified attacks on rebel-held parts of Aleppo.
Mr Kerry’s spokesman, John Kirby, says the politician expressed grave concern over the situation in Syria’s second city.
He told Mr Lavrov the US holds Russia responsible for the use of incendiary and bunker-buster bombs in an urban area, Mr Kirby said.
During the call, he said the US was preparing to “suspend US-Russia bilateral engagement on Syria,” including on a proposed counter-terrorism partnership, “unless Russia takes immediate steps to end the assault on Aleppo” and restore a ceasefire.
In the latest attacks, medical officials said an airstrike had knocked out generators and cut off the water supply at the M10 hospital – killing two seriously ill patients.
Meanwhile, another medical centre was struck by artillery fire, which killed six people queuing for bread at a nearby bakery.
Adham Sahloul, of the Syrian American Medical Society, described the attacks as deliberate.
He added: “With these two hospitals gone, if today there is another offensive like Saturday or Sunday, this is signing the death warrant for hundreds of people.”
It was unclear if the strikes were executed by Syrian or Russian forces, both of which are carrying out raids in Aleppo.
M10 radiologist Mohammad Abu Rajab said: “The rubble fell in on the patients in the intensive care unit.”
Pope Francis has warned those behind the bombing of the city will have to “answer before God”.