Russia this week sent a formal diplomatic note to the United States warning that U.S. and NATO shipments of the “most sensitive” weapons systems to Ukraine were “adding fuel” to the conflict there and could bring “unpredictable consequences.”

The diplomatic démarche, a copy of which was reviewed by The Washington Post, came as President Joe Biden approved a dramatic expansion in the scope of weapons being provided to Ukraine, an $800 million package including 155 mm Howitzers — a serious upgrade in long-range artillery to match Russian systems — coastal defense drones and armored vehicles, as well as additional portable anti-air and antitank weapons and millions of rounds of ammunition.

The United States has also facilitated the shipment to Ukraine of long-range air defense systems, including Slovakia’s shipment of Russian-manufactured Soviet-era S-300 launchers on which Ukrainian forces have already been trained. In exchange, the administration announced last week, the United States is deploying a Patriot missile system to Slovakia and consulting with Slovakia on a long-term replacement.

The 5th Battalion, 7th Air Defense Artillery’s U.S. Army Patriot Missile Systems arrived in Croatia May 17, 2021 to participate in DEFENDER-Europe 21 associated exercises Astral Knight 21 and Immediate Response 21. Shipment of the weapons, the first wave of which U.S. officials said would arrive in Ukraine within days, follows an urgent appeal to Biden from Ukrainian President Volodymyr Zelensky, as Russian forces were said to be mobilizing for a major assault on eastern Ukraine’s Donbas region 

Shipment of the weapons, the first wave of which U.S. officials said would arrive in Ukraine within days, follows an urgent appeal to Biden from Ukrainian President Volodymyr Zelensky, as Russian forces were said to be mobilizing for a major assault on eastern Ukraine’s Donbas region and along the coastal strip connecting it with Russian-occupied Crimea in the south. Russian troops have largely withdrawn from much of the northern part of the country, including around the capital, Kyiv, following humiliating defeats by the Ukrainian military and local resistance forces.

“What the Russians are telling us privately is precisely what we’ve been telling the world publicly — that the massive amount of assistance that we’ve been providing our Ukrainian partners is proving extraordinarily effective,” said a senior administration official, who spoke on the condition of anonymity about the sensitive diplomatic document.

The State Department declined to comment on the contents of the two-page diplomatic note or any U.S. response.

Russia experts suggested Moscow, which has labeled weapons convoys coming into the country as legitimate military targets but has not thus far attacked them, may be preparing to do so.

“They have targeted supply depots in Ukraine itself, where some of these supplies have been stored,” said George Beebe, former director of Russia analysis at the CIA and Russia adviser to former vice president Dick Cheney. “The real question is, do they go beyond attempting to target [the weapons] on Ukrainian territory, try to hit the supply convoys themselves and perhaps the NATO countries on the Ukrainian periphery” that serve as transfer points for the U.S. supplies.