The Chinese military said that an American naval ship had “illegally intruded” on Monday into waters near the Second Thomas Shoal, the site of a hot territorial dispute between China and the Philippines in the South China Sea.

A Chinese naval force was mobilized to track the USS Gabrielle Giffords during the operation, according to a statement from the People’s Liberation Army Southern Theater. The U.S. Navy’s 7th Fleet said the ship “was conducting routine operations in international waters ... consistent with international law.”