Japan Coast Guard officials say that so far this year, 50 Chinese patrol ships have been spotted in Japanese territorial waters near the Senkaku Islands in the East China Sea. They say the sightings occurred over a total of 14 days.
Japan controls the islands. The Japanese government maintains they are an inherent part of Japan's territory. China and Taiwan claim them.
Officials say such sightings have occurred since 2012, when Japan's government acquired ownership of some of the islands from a Japanese owner.
In 2017, a total of 108 Chinese patrol vessels entered the waters over a total of 29 days.
Coast Guard officials say if further incidents take place, they will continue to demand the vessels immediately leave Japanese waters and take firm action in accordance with international and domestic laws.