The Chinese military on Saturday said it expelled U.S. guided-missile destroyer USS Hopper after it illegally trespassed into Chinese territorial waters off the Xisha Islands in the South China Sea.

The People’s Liberation Army (PLA) Southern Theater Command has deployed its naval and air forces to “track, monitor and warn away” the vessel, said Senior Colonel Tian Junli, the spokesperson of the theater command, in a statement.

According to Tian, the U.S. has “severely jeopardized China’s sovereignty and security,” and the illegal entry serves as “ironclad evidence” of Washington’s pursuit of “navigation hegemony” and “militarization of the South China Sea.”

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