Russia issued a formal diplomatic warning to the United States to stop arming Ukraine following the sinking of a Russian flagship in the Black Sea. In issuing this demand, Moscow warned of “unpredictable consequences” for international security if its demands are left unmet.

On Thursday evening, the Washington Post reported that the Russian Embassy in Washington D.C. passed a diplomatic letter to the State Department detailing Russia’s demand to cease arming Ukraine.

Titled “On Russia’s concerns in the context of massive supplies of weapons and military equipment to the Kyiv regime", the note accused the U.S. and NATO of pressuring Ukraine to "abandon" stalled peace negotiations with Russia and keep fighting. It also accused the U.S. of acting against Russia by discouraging military cooperation with it while facilitating transfers of Soviet-era weapons to Ukrainian forces.

To date, President Joe Biden has provided the green light for billions in military aid to Ukraine. On Thursday, the administration announced that an $800 million military aid package would include items desired by Ukraine's President Volodomyr Zelensky, including armored vehicles and more missiles.

Most ominously, the letter warned that the U.S. risked creating the conditions for “unpredictable consequences for regional and international security.” The Post, whose reporters viewed a copy of the letter, did not lay out what this may entail but it raised fears about whether Russia would strike any NATO convoys entering Ukraine and risk widening the conflict.

Since the start of the war, Russia has warned against any weapons transfers to the Ukrainian military or risk being considered a target. The Russian military has launched attacks on supply depots and training sites across Ukraine, including one on March 13 that left 35 dead.

The warning also raised the specter of the war escalating into a nuclear conflict. After ordering his forces into Ukraine, Russia’s President Vladimir Putin warned the West against interfering lest they invite “consequences…never seen in history” and ordered his nuclear forces to enter full alert.

The Biden administration has been wary of entering into an escalating spiral with Putin that may lead to a nuclear exchange. On Thursday, CIA Director William Burns warned against taking "lightly" the threat of Russia using a tactical nuclear warhead in the face of military setbacks.

Russia’s warning arrived one day after Ukraine claimed its military sank the flagship of the Russian Black Sea Fleet, the Moskva, off the coast of Odesa.

Moscow claims the ship sank after a prolonged fire and in stormy conditions at sea, but Ukrainian officials say they struck the vessel with two Neptune anti-ship missiles. The Pentagon has yet to confirm if it believes the Ukrainian version of events.