Japan has lodged a "strong protest" against Russian President Vladimir Putin's visit to the disputed Kuril Islands.
Part of the island chain - referred to by Japan as the Northern Territories - is "inherently Japanese territory," the Foreign Ministry in Tokyo said.
The Foreign Ministry statement came after Russian state media reported that Putin had visited the island of Iturup - known in Japan as Etorofu - on Thursday, his first visit there.
According to the Kremlin, he visited a fish-processing plant, a hospital and a school on the island.
Iturup is one of the four southernmost Kuril Islands claimed by both Russia and Japan. They are currently under Russian administration.
The territorial dispute over the islands has strained relations between Tokyo and Moscow for decades.
The Kuril Islands are a fish-rich archipelago in the Pacific that the Soviet Union seized from Japan at the end of World War II.
Both sides have since disputed the ownership of the four southernmost Kuril Islands, and no peace treaty has been signed to this day.
Some parts of the strategically located Kurils are just a few kilometres away from the Japanese island of Hokkaido. The island group is of military significance to Russia.