The Japanese government has protested to South Korea over a visit by South Korean lawmakers to the Takeshima Islands in the Sea of Japan.
South Korea controls the islands. Japan claims them.
A group of South Korean lawmakers landed on the islands at around 11:30 AM on Saturday.
They say the purpose is to protest Tokyo's removing South Korea from its list of trading partners entitled to preferential export procedures and its handling of the issue of wartime labor.
They offered encouragement to South Korean guards and visited facilities built on the islets.
The Japanese Foreign Ministry's Asian and Oceanian Affairs Bureau Director-General, Kenji Kanasugi, called a senior official at the South Korean Embassy in Tokyo to lodge a protest.
Kanasugi said the visit was made despite Japan's protest and a demand that the visit be cancelled. He said it is totally unacceptable and extremely deplorable, given that Takeshima is an integral part of Japanese territory historically, geographically and under international law.
Japan's embassy in Seoul made a similar protest to South Korea's foreign ministry.
Earlier this week, the South Korean military carried out two days of drills in waters near the islets, and Japan lodged a strong protest.