Japan has "strongly protested" on Wednesday against Russian military exercises on Pacific islands that it also claims and which have been at the root of strained relations between the two countries since the end of World War Two.
The exercises on the disputed islands are a blow to Prime Minister Shinzo Abe's efforts to court resource-rich Russia and keep the door open to dialogue, despite the Ukraine crisis.
Russia seized the islands from Japan in the waning days of World War Two and the dispute over them has prevented the neighbours from signing a formal peace treaty.
The islands are known as the Southern Kuriles in Russia and the Northern Territories in Japan.
"Carrying out this sort of exercises in the Northern Territories is totally unacceptable," Abe told reporters.
Japan lodged a "strong protest" at the Russian Embassy in Tokyo, the Japanese Foreign Ministry said in a statement, calling the exercises "extremely regrettable".
Japan had already protested at the beginning of the military exercises but reiterated its objections on Wednesday.
Colonel Alexander Gordeyev, a spokesman for Russia's Eastern Military District, told the Russian news agency Interfax on Tuesday that the exercises had begun, involving military units in the region deployed to the islands.
Gordeyev said more than 1,000 troops, five Mi-8AMTSh attack helicopters and 100 other pieces of military hardware would be involved in the manoeuvres.
The exercises included Etorofu and Kunashiri islands, which Japan claims. It also claims Shikotan island and the Habomai islet group.
Russia is 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. Russia denies any such intent.
Abe, however, has made an effort to improve Japan's ties with Russia a key part of his diplomacy.
His government treads a fine line by imposing sanctions on Russia in step with ally the United States, but keeps them lighter than those ordered by Washington in a bid to prevent significant damage to relations with Moscow.
In his first year in office, Abe met Russian President Vladimir Putin five times, while failing to secure a summit with the leaders of neighbouring China or South Korea.
Closer ties between Japan and Russia are driven largely by energy interests.
Russia plans to at least double oil and gas flows to Asia in the next 20 years and Japan has been forced to resort to huge fuel imports to replace lost nuclear energy, after reactors were shut down because of the 2011 Fukushima disaster.