Japan's Chief Cabinet Secretary Yoshihide Suga has revealed that the government has protested a South Korean research ship's entry into Japan's territorial waters near the Takeshima Islands in the Sea of Japan.
Suga said at a news conference on Tuesday that a South Korean research vessel entered regional waters last Friday and from Sunday to Monday.
This prompted the Japan Coast Guard to issue warnings and to patrol the area.
Suga said the Japanese government has demanded through diplomatic channels an explanation from South Korea for the ship's actions. He added that if the ship is engaged in research, that activity is being done without Japan's consent and cannot be permitted.
Japan also lodged an official protest with South Korea last November, after confirming the entry of a South Korean research ship twice into waters near Takeshima.
South Korea controls the islands. Japan claims them. The Japanese government maintains the islands are an inherent part of Japan's territory. It says South Korea is illegally occupying them.