South Korea's military and maritime police began a joint defense exercise on Tuesday around a group of islets in the Sea of Japan, local media reported, with Japan protesting the move around the outcroppings it claims.
The South Korean-controlled islets are called Takeshima in Japan and Dokdo in South Korea. South Korea conducts the drill twice a year, usually in June and December.
The latest drill mainly involves maritime training, not a landing exercise, according to the media. The military reportedly does not plan to make public footage from the drill, in a bid to avoid raising tensions with Japan.
In Tokyo, Japan's top government spokesman Katsunobu Kato called the drill "unacceptable and extremely regrettable." The chief Cabinet secretary said Japan has protested against the exercise through diplomatic channels and is calling on South Korea to stop it.
South Korean media reported earlier that Japan unilaterally canceled a meeting between Prime Minister Yoshihide Suga and President Moon Jae In that had been planned during the Group of Seven summit in Britain, due to Seoul's plan to conduct the drill.
Japan has denied the report, saying that the meeting did not take place because of scheduling issues.
Ties between the two countries have sunk to their lowest level in decades over several historical and diplomatic issues, especially after the South Korean Supreme Court in 2018 ordered Japanese companies to compensate South Korean plaintiffs for wartime forced labor.