Chinese coast guard vessels have been passing by Japanese-claimed waters for weeks in the East China Sea and China's warships have been edging near Japan's southwestern islands in recent days, Japanese officials said.

A fleet of four Chinese coast guard vessels on Thursday passed just outside of the territorial waters of Japan-controlled islands, which Beijing also claims, for the 49th day in a row, the Japanese Coast Guard said.

It warned China’s vessels against further approaching the islands, called the Senkaku in Japanese, while Beijing calls them the Diaoyu. China's Coast Guard on Tuesday acknowledged that it was patrolling waters off the Diaoyu Islands, which are also claimed by Taiwan.

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China routinely sends coast guard vessels and planes into waters and airspace surrounding the islands to harass Japanese vessels in the area and force Japan to scramble jets in response.

Japan Coast Guard Commandant Shohei Ishii said last month that Chinese Coast Guard activity in infiltrating Japanese territorial waters violates international law, and that the "situation is extremely serious and is unpredictable."

Tokyo in recent years has significantly reinforced defense of southwestern Japan, including Okinawa and its outer islands that are considered strategically key to the Japanese defense in the face of China’s growing assertiveness and tension around Taiwan, an autonomous island that Beijing says is its territory.

Japan's Defense Ministry said this week that it has repeatedly spotted Chinese warships off the coast of Okinawa since the previous week. On Feb. 1, a Chinese guided missile destroyer and frigate crossed the waters between Okinawa and the Miyako islands as they moved south, causing the deployment of a Japanese Self Defense Force warship and a reconnaissance aircraft.

On Saturday, a reconnaissance ship passed the area to the north. Then, on Sunday, the Chinese reconnaissance ship showed up in the waters again, the ministry said. On Monday, the guided missile destroyer and the frigate were spotted crossing the waters between Okinawa and Miyako, the ministry said.

Japan has been increasingly concerned about China's maritime activities, including its joint military exercises with Russia around the Japanese coasts. Under a new security strategy adopted in December, 2022, Japan is accelerating the development of long-range cruise missiles that can hit targets in China or North Korea.