This undated satellite image shows the Yongshu Jiao of China's Nansha Islands. (PHOTO / XINHUA)

The Chinese military followed and monitored a United States warship that illegally entered the waters near Mischief Reef in the South China Sea, a spokesman said on Monday.

"The guided-missile destroyer USS Milius on Monday illegally entered waters near Mischief Reef in the Nansha Islands without the approval of the Chinese government," Senior Colonel Tian Junli, spokesman for the People's Liberation Army Southern Theater Command, said in a statement on Monday.

The theater command organized naval and air forces to follow and monitor the whole process of the warship, he said.

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"China has indisputable sovereignty over South China Sea islands and their adjacent waters," Tian said, stressing that the theater command troops are always on high alert to resolutely safeguard China's sovereignty and peace and stability in the South China Sea.