MOSCOW—The Kremlin said that U.S. Patriot missile-defense systems would be a legitimate target if supplied to Ukraine, signaling that Russia doesn’t intend to cease its military offensive even in the face of superior weaponry. U.S. officials this week said Washington was finalizing plans to provide Ukraine with the highly advanced weaponry that would significantly boost its air-defense capabilities, following months of Russian missile and drone attacks on the country’s power grid, cities and civilian infrastructure. Kremlin spokesman Dmitry Peskov told reporters Wednesday that the coveted missile- defense batteries would absolutely be fair game for a strike, but added that he...