The Chinese military monitored and warned off U.S. guided-missile destroyer USS Milius after it illegally trespassed into Chinese territorial waters off Xisha Islands in the South China Sea on Thursday, the People's Liberation Army (PLA) Southern Theater Command said in a statement.  

Senior Colonel Tian Junli, the spokesperson of the theater command, said the warship ventured into Chinese territorial waters without permission, which severely undermined the peace and stability in the South China Sea region.  

The theater command will remain on high alert and take all necessary measures to resolutely safeguard national sovereignty, security and the peace and stability of the region, the spokesperson added.