Russia has formally complained to the United States over its military aid to Ukraine, warning of "unpredictable consequences" if shipments of advanced weaponry go forward, the Washington Post reported on Friday.

In a diplomatic note this week, Moscow warned the United States and NATO against sending the "most sensitive" weapons for Kyiv to use in the conflict with Russia, saying such shipments were "adding fuel" to the situation and could come with "unpredictable consequences," the Post reported.

"We call on the United States and its allies to stop the irresponsible militarization of Ukraine, which implies unpredictable consequences for regional and international security," the Post quoted Russia saying in the note to the United States.

In Moscow, a Foreign Ministry spokeswoman confirmed Russia had sent diplomatic notes to the United States and other nations about supplying weapons to Ukraine but did not say what the messages contained, Interfax news agency reported.

The U.S. State Department declined to comment on reports of the formal note, AFP reported.

The warning from Russia came the same week when U.S. President Joe Biden pledged a new $800 million military aid package for Ukraine, including helicopters, howitzers and armored personnel carriers.

Biden told his Ukrainian counterpart Volodymyr Zelenskyy of the new weapons aid package over the phone Wednesday.

"This new package of assistance will contain many of the highly effective weapons systems we have already provided and new capabilities tailored to the wider assault we expect Russia to launch in eastern Ukraine," said Biden.

Zelenskyy for his part tweeted that he and Biden had discussed the "additional package of defensive and possible macro-financial aid."

The Ukrainian president has been pleading with U.S. and European leaders to provide heavier arms and equipment to help it end the siege of Mariupol and fend off an expected Russian offensive in the east.

The new U.S. assistance included some of the heavier equipment that Washington had previously refused to provide to Kyiv for fear of escalating the conflict with nuclear-armed Russia, and comes following previous weapons aid already supplied to the Ukrainian army.

(With input from agencies)