Two Chinese government surveillance ships entered Japanese territorial waters around the disputed islands in the East China Sea on Monday, the Japan Coast Guard said. The two maritime surveillance ships moved into Japan's waters for the first time in five days, at around 9:25 a.m. (0025 GMT), the coast guard said. The ships belong to China's State Oceanic Administration. The coast guard warned the Chinese ships to leave the waters, but one of the ships replied that the islands are Chinese territory. In Tokyo, the government upgraded its liaison center at the Prime Minister's Office to taskforce level after the entry by the Chinese vessels into Japanese territorial waters. Since its launch in 2010, the center has been monitoring China's air and sea activities around the Senkaku Islands, which are administered by Japan but also claimed by China and Taiwan. The small chain of uninhabited rocky islands, known as Diaoyu in China and Tiaoyutai in Taiwan, lie in rich fishing grounds and waters thought to contain large deposits of oil and natural gas. Bilateral relations between the world's second and third-largest economies have sharply deteriorated since Tokyo's nationalization of three of the five major islands in September last year through purchase from a private Japanese owner, which led to China's massive anti-Japan protests, with some cases escalating to vandalism, looting and arson targeting Japanese factories, shops and restaurants.