Beijing says it has filed a protest with Tokyo after the Chinese coast guard found a Japanese boat sailing in waters off the disputed Diaoyu Islands. The patrols were designed to exercise China\'s sovereignty over the islands, China\'s state-run Xinhua news agency reported Thursday. The island chain is claimed by both Japan and China, but Chinese Foreign Ministry spokesman Hong Lei said the islands \"are an integral part of Chinese territory and have been since ancient times.\" The vessel that China claimed was illegally sailing in its territorial waters off the islands belonged to a Japanese politician, Hong said. The Japanese vessel has already left the islands, he said.