MOSCOW, Russia (AFP) — President Vladimir Putin visited Thursday Russia’s far-eastern Kuril islands for the first time, prompting a “strong” protest by Japan which also claims the territory.

Russian-Japanese relations have long been dogged by tensions over 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.

Putin traveled 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.

Japan issued a stern condemnation.

“The Northern Territories... are an inherent part of Japan’s territory both historically and under international law, and the government of Japan strongly protests this visit,” Foreign Minister Toshimitsu Motegi said in a statement.

Ties have worsened since the full-scale conflict in Ukraine began in 2022, with Japan joining other G7 countries in sanctioning Russia and giving money and non-lethal defense 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 Japanese jets to scramble hundreds of times a year.

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 defense 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.