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The Defense Agency said Friday the intrusion of a Chinese submarine into Japan’s territorial waters was a highly provocative act by the Chinese Navy.

Agency officials speculated that the Chinese military was trying to use the brief trespass Wednesday to show off its sea power.

They said it was probably an attempt to intimidate Japan and help Beijing’s efforts to secure greater resources in the East China Sea, as well as to keep in check the U.S. forces that watch over Taiwan.

Military experts have said the Chinese military, which long has been largely reliant on ground forces, has been throwing its efforts behind the modernization of its navy since the late 1990s.

But agency officials are puzzled as to why the Chinese Navy, which has been expanding its activities on the high seas in recent years, chose to act in a way that was certain to invoke strong anger from Tokyo.

The sub did not appear to be hiding; it was navigating in shallow waters of 300 meters and was apparently an old Han-class nuclear-powered sub, known to be noisier than newer models. They said the Maritime Self-Defense Force found it relatively easy to track.

“It is obvious from the sub’s movements that it was challenging the MSDF’s capabilities. But we can’t be sure of China’s real intentions unless we ask,” one agency official said.

The MSDF this year has spotted an increasing number of Chinese scientific research ships and military vessels operating inside Japan’s economic exclusive zone in the East China Sea.

From January through Wednesday, patrols sighted 10 navy vessels and 16 research ships. In 2003, it said it found seven research vessels and one submarine.

Beijing and Tokyo agreed in 2001 to give each other advance notice when their research ships plan to navigate within their respective EEZs. But China has continued to send its vessels without any notice, insisting that the economic zone boundaries that Japan has set are invalid.

Defense Agency officials said they consider Wednesday’s intrusion a sign that the Chinese military has gone one step further because the sub — the first sighted in the area this year — moved into Japan’s territorial waters.