Two Chinese patrol ships detected a Japan Coast Guard vessel in the waters near the Diaoyu Islands during a routine patrol on Friday, according to a statement by the State Oceanic Administration.
But the Japanese ship didn't respond to an inquiry sent by the Chinese ships and the two Chinese ships followed the Japanese ship to track its movement, the agency said.
The statement said the patrol showed China's indisputable sovereignty over the Diaoyu Islands and its adjacent islets in the East China Sea.
Liu Weimin, spokesman of the Ministry of Foreign Affairs, said on Friday at a news conference that patrols in waters near the Diaoyu Islands and its affiliated islets are to safeguard China's marine interests.
Liu said on Tuesday that any unilateral move taken by Japan toward the area is illegal and invalid.
China released standard names and descriptions of the Diaoyu Islands and its 70 affiliated isles on March 3 on the website of the State Oceanic Administration. That was in response to the Japanese release on March 2 of the names of 39 uninhabited islands, including those of some affiliated to the Diaoyu Islands.
Huo Jiangang, an expert on Japanese studies at the China Institutes of Contemporary International Relations, said patrols clearly demonstrate China's sovereignty, although solving the disputes with Japan over this long-standing issue will not be easy.
"The win-win solution now is putting aside differences and developing the area together as Beijing proposed, but Tokyo hasn't positively responded to it," he said.
Zhou Yongsheng, a professor of Japanese studies at China Foreign Affairs University, said ongoing patrols proved China's legal and effective presence in the region, despite possible tension between China and Japan in the short run.
China has recently been strengthening its marine surveillance to safeguard its marine interests.
Another 36 patrol ships are expected to join the China Marine Surveillance fleet in the coming years.
In 2011, marine surveillance ships conducted 227 patrols, traveling about 500,000 kilometers.
Planes have carried out 354 patrols over a total distance of about 410,000 kilometers, the State Oceanic Administration said in another report on Friday.
Routine patrols last year detected and supervised illegal oil and gas exploration, marine surveying and military reconnaissance carried out by foreign countries.
With China's island protection law having come into effect in March 2010, the China Marine Surveillance fleet carried out routine patrols on the country's 10,679 islands.
In 2011, the illegal use of islands was detected in 67 cases and total fines of 724,200 yuan ($115,000) were handed down.
To enhance its offshore law enforcement, the administration added 10 patrol ships last year, according to the report.
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