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The Japanese defense minister, Itsunori Onodera, said that a Chinese navy frigate had directed its fire-control radar at a Japanese destroyer in the incident on Jan. 30 near the islands in the East China Sea, known as the Senkaku in Japan and Diaoyu in Chinese. The uninhabited island group has been controlled by Japan for decades, but claimed by China and also Taiwan.

On Tuesday, the Defense Ministry also disclosed that a Chinese frigate had directed the same kind of radar at a Japanese military helicopter in a previously undisclosed incident on Jan. 19. In both cases, the Chinese ships eventually turned off the radar without actually firing a shot.

Still, Japanese officials said the use of such radar was a threatening gesture that represented an increase in tensions, which have been growing since the Japanese government announced last summer that it would buy three of the five islands. The Chinese responded by sending paramilitary surveillance ships into or near Japanese-claimed waters around the islands on an almost daily basis. Those incursion were intercepted by Japanese coast guard ships in a high-seas game of cat and mouse.

The row grew more heated in December when Chinese surveillance aircraft began flying near the islands. Tensions rose another notch last month, when Japan and China both scrambled fighter jets that briefly monitored each other.

Still, the recent radar incidents are among the first to involve naval warships from both nations, which had until now been kept in the background to avoid a dangerous escalation. With tensions so high, military experts in Japan and the United States say their biggest fear is some accident or miscalculation resulting in an unintended military confrontation.

"One step in the wrong direction could have pushed things into a dangerous situation," Mr. Onodera told reporters.

The Chinese incursions are seen by Japanese political leaders and experts as part of a new strategy to press Japan into officially acknowledging that a territorial dispute exists, something Tokyo has so far refused to do. They also say that by maintaining a nearly constant presence, China hopes to undermine Japan's claims to be in sole control of the islands.

Japan has responded by stepping up its own surveillance, which includes keeping a small flotilla of coast guard ships near the islands, which are between Okinawa and Taiwan.

The purchase of the islands in September set off violent protests in China, where the islands are seen by many as the last pieces of Chinese territory to remain in Japanese hands from its foray into empire-building in the late 19th and early 20th centuries. Japan says that China only showed interest in the islands after undersea oil and natural gas deposits were discovered nearby in the last 1960s.