Russia began military exercises in the Kuril Islands in the Pacific Ocean, a Defense Ministry spokesman said on Tuesday, a move likely to anger Japan, which also lays claim to them.
"Exercises began involving military units in the region, which have been deployed to the Kuril Islands," Colonel Alexander Gordeyev, a spokesman for Russia's Eastern Military District, told news agency Interfax.
He said more than 1,000 troops, five Mi-8AMTSh attack helicopters and 100 other pieces of military hardware would be involved in the maneuvers.
A Japanese foreign ministry official in Tokyo said the ministry was checking whether the exercises were taking part on islands that Japan considers its territory. The islands are known as the Southern Kurils in Russia and the Northern Territories in Japan.
"If they are conducting a military exercise on the Northern Territories, we can by no means accept that in light of Japan's stance on the islands. We've already informed the Russian side of that stance and asked for clarification," the official said.
The dispute has strained relations between Japan and Russia since World War II, when Soviet forces occupied the four islands at the southern end of the Kuril chain. Japan says the islands are part of its territory and wants Moscow to hand them over.
Russia is also at odds with Western powers over what NATO says is its massing of military forces along the border with Ukraine for a possible invasion to boost pro-Russian separatists in the country's east. Moscow denies any such intent.