Japan has lodged a protest with Russia over military drills in the waters near Kunashiri, an island that is part of the Northern Territories. Russia controls the four islands. Japan claims them.
The Japanese government maintains the islands are an inherent part of Japan's territory. It says the islands were illegally occupied after World War Two.
Japan's Foreign Ministry says Russia told them on Tuesday that its forces will conduct the drills from April 4 to 12 in waters east of southern Kunashiri.
Japan's government told Russia's foreign ministry through its embassy in Moscow that the military drills are incompatible with its position on the islands.
Chief Cabinet Secretary Yoshihide Suga told reporters on Thursday that the government believes it is important to resolve the issue over the islands.
He said the government will continue to tenaciously work in line with its basic principle of settling the territorial issue and concluding a peace treaty.
Russia considers the four islands as an important strategic point and has frequently held military drills on the islands and in waters around them.
Just last month, Russia conducted military drills involving about 500 troops as well as tanks on Kunashiri and Etorofu, another island in the Northern Territories.