KYIV, Ukraine (AP) — Russia's Foreign Ministry warned Thursday that if the U.S. delivers sophisticated air defense systems to Ukraine, those systems and any crews that accompany them would be a "legitimate target" for the Russian military, a blunt threat that was quickly rejected by Washington.

The exchange of statements reflected soaring Russia-U.S. tensions amid the fighting in Ukraine, which is now in its 10th month.

Russian Foreign Ministry spokeswoman Maria Zakharova said the U.S. had "effectively become a party" to the war by providing Ukraine with weapons and training its troops. She ad...