South Korea has begun two days of war games near a set of disputed islands that are also claimed by Japan.
The two countries have long disagreed on who has rightful claim to the islands known as Dokdo in Korean and Takeshima in Japanese.
The islands are "obviously an inherent part of the territory of Japan," Kenji Kanasugi, the director general of Japan's foreign ministry's Asian and Oceanian Affairs Bureau, told the South Korean Embassy in Tokyo in a statement.
The drills began Sunday, just days after Seoul decided to scrap a military intelligence sharing agreement with Tokyo.
The latest flare-up in tensions between the two Asian nations is rooted in Japan's brutal 1910-1945 occupation of the Korean peninsula. A major source of friction is how to compensate those forced into labor and sexual slavery in the colonist era.
Japan says the reparation issue was resolved with a 1965 treaty that normalized Japan - South Korea relations.
Japan has complained that subsequent South Korean governments have not accepted further Japanese apologies and attempts to make amends.