The two incidents mark the first times during the six-month maritime standoff that China has supposedly used such radar on Japanese ships or aircraft. According to Japan’s Asahi Shimbun newspaper, which cited Japanese Defense Ministry officials, the Chinese vessel and the Japan Maritime Self-Defense Force ship were less than two miles apart when the radar was used.

Although Tokyo and Beijing have long been at odds over the rocky chain of remote islands — the countries have competing narratives about decades- and centuries-old claims — tensions spiked in September when Japan bought three of the islands from a private landowner. Previously, Japan rented and controlled the islands, although China and Taiwan also staked claims.

Since the purchase, relations between China and Japan have hit their lowest point in decades. China has steadily increased its surveillance of the waters around the islands; incursions into Japanese territory, once rare, now occur almost daily.

More recently, China has been sending aircraft to the area, and on one occasion, China and Japan scrambled fighter jets in a game of cat-and-mouse. Both governments, hemmed in by nationalism, are limited in their ability to calm the situation, security analysts in Asia say.

China’s government describes its right to the islands as an “undeniable fact backed up by historical records,” and analysts say Beijing is pressing Tokyo to acknowledge that the territory is up for grabs. Japanese officials refuse to admit that a territorial dispute even exists; they say the islands are an “inherent” part of Japan, based on “historical facts,” according to a Foreign Ministry statement.