Sept. 21 (UPI) -- Four Chinese coast guard vessels sailed into Japanese territorial waters on Thursday, near the disputed Senkaku Islands, also known as the Diaoyu Islands in China.
Japanese television network NHK reported Thursday the boats trespassed into Japanese waters at 10 a.m., and Japan's coast guard responded with warnings and "surveillance."
The boat incursion took place the same day Japan officially registered the islands as the Senkakus, the Sankei Shimbun reported.
Tokyo's coast guard also said the boats took turns sailing near Uotsurishima, an island in the Senkaku chain, for more than an hour, eventually leaving the area at about 11:30 a.m.
At around noon, the boats were seen in waters about 20 miles from Uotsurishima, but returned to Chinese waters, according to NHK.
Chinese boats have trespassed into Japanese territorial waters 23 times in 2017, according to Tokyo.
On Tuesday, a group of four Chinese guard boats entered Japanese waters without permission in the evening, and continued to sail in territorial waters until the following morning.
In early August, two Chinese coast guard vessels set off warnings when they appeared in Japanese territorial waters near Kagoshima Prefecture in southern Kyushu.
Japan's registration of the Senkaku Islands as its separate administrative district may be a response to increased Chinese activity in the area.
The move to change the district's name from Tonoshiro to Senkaku is being led by Yoshitaka Nakayama, the mayor of Ishigaki Island, a Japanese territory close to Taiwan.
Tokyo has said 643 ships of Chinese origin have entered territorial waters in the last 199 days.
The United States is obligated to defend Japan, including the Senkakus, in the event of foreign attacks.