The long-running territorial dispute between Japan and Russia over the Kuril Islands has once again come into focus after Russian President Vladimir Putin made his first visit to the disputed territory.

Japan reacted strongly to the visit, with Prime Minister Sanae Takaichi calling Putin’s actions “absolutely unacceptable.” Foreign Minister Toshimitsu Motegi also summoned Russian Ambassador Nikolay Nozdrev over the visit.

The islands are known in Japan as the Northern Territories and in Russia as the Southern Kurils. They have been claimed by both countries for decades, and the dispute has prevented Japan and Russia from signing a formal post-World War II peace treaty.

The dispute involves four islands — Etorofu, Kunashiri, Shikotan and the Habomai islet group — located just off Japan’s northern island of Hokkaido.

The islands were seized by the Soviet Union during the final stages of World War II. The Soviet Union declared war on Japan in August 1945, as Tokyo was preparing to surrender, and took control of the islands.

Around 17,000 Japanese people formerly lived on the islands before being deported after Russia took control. Today, around 20,000 Russian citizens live there. Japan continues to maintain that the islands are part of its territory. Takaichi said the disputed islands were “both historically and under international law” part of Japan.

She also said Putin’s visit was incompatible with Japan’s consistent position on the territory.

The islands have strategic importance for both countries and provide access to rich fishing grounds and valuable mineral deposits. Putin visited the island of Etorofu, known as Iturup in Russia, where he visited a hospital, met local residents and tried caviar at a processing plant.

Japanese television also showed comments from former Japanese residents who were deported after Russia took control of the islands.

Putin had travelled to the nearby Russian island of Sakhalin on Wednesday, where he inspected naval drills. He also said the four disputed islands were recognized as Russian in international documents.

The visit marked Putin’s first trip to the disputed islands. Former Russian President Dmitry Medvedev had visited Kunashiri in 2010 while he was president and Putin was prime minister.

Moscow strongly rejected Japan’s criticism of Putin’s visit. Russian Foreign Ministry spokeswoman Maria Zakharova described statements from Takaichi and Motegi as “politically irrelevant, offensive” and “legally void.” She said Japan had no grounds to tell Russia’s leadership which regions of the country its president could visit.

Zakharova also accused Tokyo of pursuing an increasingly anti-Russian policy, pointing to Japan’s participation in Western sanctions against Russia and its military and other support for Ukraine.

Russia maintains that its sovereignty and jurisdiction over the southern Kuril Islands, including Iturup, are undisputed. Zakharova said the islands became part of Russia following World War II and accused Japan’s leadership of promoting “revanchist” narratives.

Relations between the two countries had previously shown signs of improvement under former Japanese Prime Minister Shinzo Abe, who reached an understanding with Putin that allowed visits by Japanese citizens to family graves and revived discussion of a possible peace treaty.

However, Russia’s 2022 invasion of Ukraine led Japan to impose sanctions on Russian personnel and entities, further souring relations.

The Kuril Islands dispute therefore remains unresolved, with both Tokyo and Moscow continuing to assert competing claims over the territory.