10 Chinese government surveillance ships were spotted near Japanese territorial waters off the disputed islands in the East China Sea on Thursday, the Japan Coast Guard said. Since Tuesday, a total of 16 Chinese official surveillance vessels, including six fisheries surveillance ships, have entered the contiguous zone adjacent to Japanese territorial waters around the Senkaku Islands, which are administered by Japan but also claimed by China and Taiwan, marking China\'s biggest-ever intrusion into the area. The six boats were confirmed to have left the area on Wednesday night, said the coast guard, which continue warning the Chinese ships not to enter Japanese territorial waters by radio and other means. The coast guard also said it will increase its number of patrol ships to continue close monitoring of the area. 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. The intrusion is widely seen as a retaliatory measure from China against the nationalization by the Japanese government of the islets on September 11. Since then, massive anti-Japan protests, with some cases escalating to vandalism, looting and arson targeting Japanese factories, shops and restaurants, also erupted across China until Tuesday. In addition, China\'s state-run radio reported on Monday that nearly 1,000 fishing boats were heading for the Senkaku Islands. Japan claimed the disputed uninhabited island chain as official Japanese territory in 1895. The islets came under US control after World War II but were returned to Japan in 1972. A dispute over the islands among three countries has been brewing ever since a United Nations Economic Commission for Asia and the Far East report suggested in 1968 that there are oil deposits under the East China Sea.