TOKYO -- Japan has asked the Chinese government to explain why Chinese ships have strategically placed several buoys in the East China Sea near a group of disputed islands, a Japanese government spokesman said on Friday.
The spokesman, Chief Cabinet Secretary Yoshihide Suga, told reporters that ships from China's State Oceanic Administration, which is similar to the coast guard, had placed the buoys last week in Chinese-controlled waters near the islands, known as the Senkaku in Japan and Diaoyu in China. The uninhabited islands have been controlled by Japan for decades, but are claimed by China and also Taiwan.
Japanese media reported that the buoys might be used to track Japanese submarines in waters around the uninhabited islands, where Japanese and Chinese ships have chased each other in recent months. If so, their placement could represent another step in an ominous escalation in the standoff, which began with coast guard and other nonmilitary ships, but has recently begun to involve more heavily armed navy ships.
Tensions over the islands flared up in September, after the Japanese government announced that it would buy three of the five islands from their private owner, setting off violent street protests in China. The Chinese government responded by sending oceanic administration and other nonmilitary ships into Japanese-claimed waters on almost a daily basis.
Earlier this month, tensions seemed to rise when Japan said that a Chinese navy frigate had briefly used a missile-directing radar to make a target of a Japanese military ship. China has denied doing that.
Mr. Suga did not say how far the buoys were located from the islands. He said they were in undisputed waters controlled by China, but had been placed on Feb. 17 less than 1,000 feet from the edge of Japanese-controlled waters.
He said his government had asked China for an explanation, saying it was also possible that the buoys were being used to track ocean currents or weather. However, the Japanese defense minister, Itsunori Onodera, told reporters that the buoys may be used to track nearby vessels.
The sparring over the islands came as South Korea criticized Japan for sending a top government official to ceremonies highlighting Japan's claim to another set of islands, which are claimed by South Korea. In a statement, the South Korean Foreign Ministry said, "We strongly protest the Japanese government's decision to send a government official to such an unjustifiable event."
This article originally appeared in The New York Times.