US media have reported that Russia sent a formal diplomatic note to the United States warning of ''unpredictable consequences'' if Washington continues to send arms to Ukraine.

The Washington Post reported in its online edition on Thursday that it had reviewed the document, which was dated Tuesday.

It said that the letter, written in Russian with a translation, was entitled "On Russia's concern in the context of massive supplies of weapons and military equipment to the Kiev regime".

Russia has reportedly accused the US and other NATO members of violating "rigorous principles" governing the transfer of weapons to conflict zones.

Moscow is also said to have accused them of being oblivious to "the threat of high-precision weapons falling into the hands of radical nationalists, extremists and bandit forces in Ukraine."

The reports say Russia called on the US and its allies "to stop the irresponsible militarization of Ukraine, which implies unpredictable consequences for regional and international security."

CNN television quoted a source familiar with the Russian diplomatic note as saying that it could signal a more aggressive Russian posture towards the US and NATO as the war continues.