China's Defense Ministry has vowed to bolster its "combat readiness" to defend against what it said was a "serious infringement" of the country's sovereignty after the U.S. Navy dispatched two warships for an apparent "freedom of navigation" operation (FONOP) in disputed South China Sea waters.

The ministry said late Sunday that the Chinese military had warned the two U.S. warships to leave after they entered waters near the contested Paracel Islands in the strategic waterway.

The two warships, the USS Antietam, a guided-missile cruiser home-ported in Yokosuka, Kanagawa Prefecture, and the USS Higgins, a destroyer, had "arbitrarily entered China's territorial waters around the Xisha Islands without permission of the Chinese government," spokesman Wu Qian said, using the Chinese name for the Paracels.