Russia has formally warned the US and other allied nations against supplying weapons to Ukraine.
The warning came in a formal diplomatic note from Moscow, a copy of which has been reviewed by media outlets in the US, BBC reported.
The two-page diplomatic note - forwarded to the US State Department by the Russian embassy in Washington - warns that US and NATO weapons shipments are "adding fuel" to the conflict in Ukraine, and could lead to what Russian diplomats refer to as "unpredictable consequences", BBC reported.
It was sent on Tuesday, just as word of a new US military aid package for Ukraine had started to leak out. Only hours later President Biden approved the shipment of $800m of military assistance - including, for the first time, long range artillery weapons such as howitzers - with the aim of matching Russia's military capability in Ukraine.
A senior US administration official was quoted as saying the warning could be seen as a concession by Russia that US and NATO military assistance to Ukraine was proving effective.
The first part of this latest shipment is expected to arrive in Ukraine in the next few days, as Russian forces continue to mobilise in the east of the country, ahead of what is expected to be a major assault on the disputed Donbas region of Ukraine in the next few weeks.
Since the war began, the US has supplied more than $3bn in military assistance to Ukraine.