Japan said it "strongly protests" Russian President Vladimir Putin's visit to a disputed island chain.
Russian state media reported that it was Putin's first visit to Iturup (Etorofu), one of the four islands in the Kuril chain claimed by both Russia and Japan.
Foreign Minister Toshimitsu Motegi said the islands are inherently Japanese territory, both historically and under international law, and the Government of Japan strongly protests this visit.
The dispute has prevented Moscow and Tokyo from signing a peace treaty after World War II.
The Kuril islands sit between Hokkaido in northern Japan and Russia's Kamchatka Peninsula; the four southernmost islands are claimed by Japan, which calls them the Northern Territories.
They are administered by Russia. At the end of World War II the Soviet Union annexed the island chain and deported around 17,000 Japanese residents.
During the visit, Putin sampled fish roe and spoke to local residents, state media said.
The islands are strategic for Russia's military; Iturup hosts Russia's primary military air base in the region.
Last month Chinese and Russian naval warships sailed through a strait during a joint patrol.
Russia also held military drills near the islands last year, drawing condemnation from Japan.
