China's "aggressive behaviour" in the South China Sea, including the use of water canon by its coast guard against a Philippine vessel, must be challenged and checked, the commander of the U.S. Navy's Seventh Fleet said on Sunday. Vice Admiral Karl Thomas assured the Philippines of U.S. backing in the face of "shared challenges" in the region, saying: "My forces are out here for a reason." The largest of the U.S. Navy's forward-deployed fleets, the Seventh Fleet, headquartered in Japan, operates as many as 70 ships, has around 150 aircraft and more than 27,000 sailors.