U.S. says to end Syria work with Russia if Aleppo assault goes on

(L-R) U.S. Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov pose for a photo before a Middle East Quartet Principals Meeting during 71st Session of the United Nations General Assembly in Manhattan, New York, U.S., September 23, 2016. REUTERS/Andrew Kelly

WASHINGTON (Reuters) - The United States threatened on Wednesday to halt its diplomacy with Russia on Syria and it said it held Russia accountable for the use of incendiary bombs on Aleppo - a view a U.S. 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.