use of such radar was a threatening gesture that signified an increase in tensions, which have been growing since Japan’s government announced last year that it would buy three of the five uninhabited islands from a Japanese citizen. Japan has controlled the islands for decades. The Chinese responded to that move by sending paramilitary surveillance ships almost daily into or near Japanese-claimed waters around the islands, where they engage in cat-and-mouse manoeuvres with Japanese Coast Guard ships.

On Monday, a senior Chinese military commander, Qi Jianguo, said protecting China’s “core national interest" at sea was an “unshakable mission." He added, “No country should underestimate the staunch will of the Chinese nation to defend its national sovereignty."

The dispute between the countries intensified in December when Chinese surveillance aircraft began flyingnear the islands. Tensions rose another notch last month, when Japan and China scrambled fighter jets.

The most recent reported radar action would be 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 exchange of fire.

The Chinese incursions are seen by Japanese political leaders and analysts as part of a new strategy to press Japan into officially acknowledging that a territorial dispute exists. 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