Japan on Thursday "strongly" protested Russian President Vladimir Putin's visit to disputed Etorofu Island, according to local media.
Japan's 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," Tokyo-based Kyodo News reported.
Putin visited Etorofu Island on Thursday, in his first trip to the Russian-controlled, Japanese-claimed island chain off Hokkaido.
The two countries have been at odds over the islands for decades, and it is the primary reason Moscow and Tokyo did not sign a peace treaty following World War II.
Tokyo maintains that the Northern Territories, consisting of Etorofu, Kunashiri, Shikotan and the Habomai islet group, were "illegally" seized by the now-defunct Soviet Union soon after Japan's surrender in World War II on Aug. 15, 1945.
Russia, which calls them the Southern Kurils, argues the seizure was legitimate.
