Russia has warned the United States not to supply arms to Ukrainian government forces fighting pro-Russian separatists in eastern Ukraine.
Russia's foreign ministry spokesman Alexander Lukashevich said a US official's suggestion that Washington should send arms to Kiev sent a "very serious signal" according to a report by Reuters news agency.
The foreign ministry warned against a "major shift in policy of the (US) administration in regard to the conflict" that would constitute a "direct violation of agreements reached, including with the participation of the United States".
The spokesman made the comments as US Vice President Joe Biden was set to arrive in the Ukrainian capital Kiev.
Washington has thrown its weight behind Kiev as it battles a pro-Russian insurgency in the east of the country. For its part, Moscow has backed the separatist rebels, although it denies sending weapons and forces to fight its neighbours.
The United Nations said more than 4,300 people have been killed in the conflict since April.
The incoming no 2 at the State Department, Anthony Blinken, told a congressional hearing on Wednesday that sending weapons to Ukraine could cause Russia to back down in eastern Ukraine.
"One thing that could hopefully get them to think twice and deter them from further action is strengthening the capacity of the Ukrainian forces, including with defensive lethal equipment."
The subject will be discussed between Biden and Ukraine's leadership in Kiev, Blinken added.