The United States decision to send more military aid to Ukraine "increases the danger of a direct military clash" between Russia and the West, Moscow has warned, reports BBC.
Russia's ambassador to the US, Anatoly Antonov, said it was an "immediate threat" to Moscow, describing the US as "a participant of the conflict".
In a statement on Thursday, Antonov warned that the US decision "to continue pumping the Kiev regime with heavy weapons only secures Washington's status as a participant of the conflict".
He said this would result in "protracted bloodshed and new casualties".
The Russian ambassador said, "We call on Washington to stop its provocative actions that could lead to the most serious consequences."
Earlier, the US announced another $625m (£547m) in military aid to Ukraine.
Advanced US weaponry has been credited with helping Ukraine build momentum against occupying Russian forces.
Ukrainian troops have made significant advances in the country's north-east and south in recent weeks.
The latest US hardware includes another four of the high-precision Himars multiple rocket systems.
Bd-pratidin English/Golam Rosul
