Russia's President Vladimir Putin attends a meeting with the Sakhalin region governor on Iturup island, one of four islands in the contested chain known as the Kuril Islands in Russia and the Northern Territories in Japan, on August 13, 2026. [AFP]

President Vladimir Putin visited Russia's far-eastern Kuril islands for the first time on Thursday, prompting protests by Japan which also claims the territory.

Russian-Japanese relations have long been dogged by the contested islands, known in Japan as the Northern Territories and seized by the Soviet Union in 1945.

The Kremlin posted a video of Putin on one of the Kurils' four main islands, with Russian media saying he visited a fish-processing plant and was given caviar.

It was the first visit by a Russian president since Dmitry Medvedev in 2010.

"The Northern Territories are an inherent part of Japan's territory, both historically and under international law," Japanese Prime Minister Sanae Takaichi told reporters.

Putin's visit "offends the sentiments of the Japanese people and is absolutely unacceptable," she said, adding that it "made it more difficult to restore relations in the medium to long term."

Putin travelled to the Kurils after visiting the nearby Russian island of Sakhalin on Wednesday, where Russia's navy was conducting drills.

In Sakhalin, Putin, 73, held a meeting "on ensuring the security of Russia's eastern borders," RIA Novosti said.

Also on Thursday, Japan's Foreign Minister Toshimitsu Motegi summoned Russian ambassador to Japan Nikolai Nozdrev to lodge a "strong protest" against Putin's surprise move.

Motegi told the ambassador in the meeting that Putin's visit would "have an extremely negative impact on Japan-Russia relations" already complicated by Moscow's invasion of Ukraine, according to the ministry's statement.

Ukraine sanctions

Ties between Japan and Russia -- which fought a war in 1904-5 -- have worsened since the full-scale conflict in Ukraine began in 2022.

Japan has joined other G7 countries in sanctioning Russia and giving money and non-lethal defence aid to Kyiv.

Putin on Wednesday called Japan's sanctions "unprovoked".

"Japan has identified Russia as a main source of threats. I would like to point out that we are not threatening Japan... On the contrary it has territorial claims against our country," Putin said.

Russia and China have also moved closer, with their navies and air forces conducting joint exercises in areas around Japan, promoting frequent scrambles by Japanese jets.

North Korea, which this month termed Japan's military build-up a "transformation into a war state", has also sent troops and artillery shells to help Russia's war against Ukraine.

Japan's latest defence white paper issued this month again highlighted growing risks from all three countries.

The New York Times reported in July that Russia had turned Japan into a "den of spies" and a key source of dual-use technology needed for its war in Ukraine.

Harsh climate, few people

The Soviet Union took the strategically located volcanic archipelago north of Hokkaido in the final days of World War II, and has maintained a military presence there ever since.

The contested isles are part of the volcanic Kuril archipelago, which runs in a broad arc from Russia's Kamchatka peninsula down to Japan's main northern island Hokkaido.

Their population of about 20,000 Russian citizens lives in a harsh climate, but the islands have mineral deposits including gold and silver, and rich fishing grounds.

After the islands were occupied in 1945, Moscow deported their Japanese population of about 17,000 people.

The islands then acquired strategic importance, providing air and sea bases during the Cold War as the Soviet Union confronted the United States and Japan.

"The Kurils have been fortified with Bastion coastal defence missile systems and SU-35 fighter aircraft. These provide Russia with instruments of control over sea lanes in the region, as well as leverage against Japan's claim to the islands," said Yee Kuang Heng, a professor at the Graduate School of Public Policy at the University of Tokyo.

"Putin's visit may be a reminder of the cards he holds and also his displeasure with Japan's continuing sanctions on Russia imposed after the invasion of Ukraine," Heng told AFP.