The Japanese government notified Russia via diplomatic channels that Russia’s military exercises that began on Tuesday in the south of the Kuril Archipelago were inadmissible, a Japanese Foreign Ministry source told on Wednesday.

“The diplomatic representation was made at the level of a counselor of our country’s embassy in Moscow and was addressed to the Russian Foreign Ministry,” the official said. “The drills, as was already stated, will help to strengthen the Russian military contingent on the Northern Territories. This contradicts our country’s stance regarding those islands and is inadmissible.”

The Russian military on Tuesday launched an exercise on Etorofu and Kunashiri, two of the four Russian-held islands off Hokkaido claimed by Japan, according to the Interfax news agency.

Col. Alexander Gordeyev, a spokesman for Russia’s Eastern Military District, said that the district’s motorized rifle units began an opposing force exercise on disputed islands.

According to The Japan Times, Some 500 troops from the machine gun and artillery units participated in the drill, including practicing the prevention of enemy landings, the Russian news agency said.

The exercise involved some 70 types of military equipment, including T-72 main battle tanks and mortars.

Moscow is strengthening its military capabilities on the four disputed islands, treating them as a major military foothold in Russia’s Far East.

Last week, the eastern district said it conducted a drill involving more than 350 troops on the Kuril Islands, which comprise an island chain called Chishima in Japan, and the four disputed islands.