China said this weekend it staged maritime defense drills involving a total of 14 patrol boats, landing craft and submarine hunting boats in the South China Sea, the latest sign of escalating tensions over the disputed waters, the Wall Street Journal reported. The announcement came as Philippine President Benigno Aquino III warned China not to intrude into waters claimed by Manila, and one day after Chinese state media reported that Beijing had dispatched once of its biggest civilian maritime patrol ships to the South China Sea. Vietnam also conducted live-fire drills in the sea four days earlier. Tensions over the sea, which contains valuable shipping routes and reserves of oil, gas and fish, began to rise last year, as China began to take a more assertive stance over the territory. This has prompted Vietnam and other Southeast Asian nations to shore up ties with the US. Malaysia, Taiwan and Brunei also claim territory in the South China Sea.