Japan's government has lodged a protest through diplomatic channels against Russia's military drills on the islands of Etorofu and Kunashiri. The two islands are among 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.
On Tuesday, the Russian military began exercises involving about 500 soldiers as well as tanks and a number of other kinds of military hardware.
The Japanese government told the Russian Foreign Ministry through the Japanese embassy in Moscow that the exercises will lead to a military buildup on the islands.
It said the exercises are incompatible with Japan's position on the islands and are therefore unacceptable.
Russia sees the islands as a point of strategic importance and has repeatedly held military drills on and near them.
Japan's Foreign Ministry plans to keep close watch over the developments.