Secretary of State John Kerry is threatening to cut off all contacts with Moscow over Syria, unless Russian and Syrian government attacks on Aleppo end.
The State Department says Kerry issued the ultimatum in a Wednesday telephone call to Russian Foreign Minister Sergey Lavrov.
Kerry's spokesman, John Kirby, says Kerry expressed grave concern over Russian and Syrian government attacks on hospitals, water supplies and other civilian infrastructure in Aleppo.
He says Kerry told Lavrov that the U.S. holds Russia responsible for the use of incendiary and bunker-buster bombs in an urban area.
Kerry told Lavrov the U.S. was preparing to "suspend U.S.-Russia bilateral engagement on Syria," including on a proposed counterterrorism partnership, "unless Russia takes immediate steps to end the assault on Aleppo" and restore a cease-fire.