Japan lodged a protest with China on Monday after four Chinese government ships entered Japanese territorial waters around a chain of disputed islands in the East China Sea. The foreign ministry conveyed the protest to the Chinese Embassy in Tokyo over the intrusion and demanded the vessels to leave Japanese waters immediately. The Japan Coast Guard said it spotted four Chinese Coast Guard vessels in the morning in the sovereign waters off the Japan-administered Senkaku Islands, which are also claimed by China and Taiwan. The coast guard warned the Chinese ships to leave the waters immediately. \"It is extremely regrettable that the Chinese official ships have intruded Japan\'s waters frequently,\" Chief Cabinet Secretary Yoshihide Suga told a press conference. 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. 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 2012 through purchase from a private Japanese owner.