Chinese navy spokesperson Liang Yang made the comments following a U.S. warship's entering waters near the Nansha Islands on Tuesday.
The Chinese navy monitored, tracked and issued warnings to the USS Lassen, according to Liang, who said China's reaction is necessary, legitimate and professional.
"China's sovereignty over the Nansha Islands and their adjacent waters is irrefutable," he said. "The Chinese navy will resolutely perform duties and missions to unswervingly safeguard national sovereignty, maritime rights and interests, and peace and stability in the South China Sea."
The spokesperson said the navy will closely monitor the situation in and above the sea for goings-on that may jeopardize China's national security.