The United States has threatened to halt its diplomacy with Russia on Syria if it does not take "immediate steps to end the assault on Allepo".
The US also said it held Russia accountable for the use of incendiary and bunker-busting bombs on Aleppo - a view a US official said could open Moscow up to charges of war crimes.
Secretary of State John Kerry conveyed the message in a call to Russian Foreign Minister Sergei Lavrov in which he voiced "grave concern" over the Russian-backed Syrian government's air and land assault on rebel areas of Aleppo, Syria's largest city.
"Secretary Kerry expressed grave concern over the deteriorating situation in Syria, particularly for continued Russian and Syrian regime attacks on hospitals, the water supply network, and other civilian infrastructure in Aleppo," State Department spokesman John Kirby said in a written statement.
"The Secretary 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 great risk," he added.
Mr Kerry also told Lavrov Washington was preparing to end its diplomatic engagement with Moscow on Syria - including possible intelligence-sharing - if Russia did not take "immediate steps to end the assault on Aleppo" and restore a defunct ceasefire.
A US official said that the use of incendiary and bunker busting weapons in an area with civilians could open Russia up to accusations of war crimes but said there is no forum in which Russia might realistically face such charges.
"The use of such weapons in a populated area in a place civilians are known to be present could be viewed as an indiscriminate attack and thus a war crime," said the official.
Reuters
