The four islands, occupied by Russia during World War II, have gained importance since the war's end, providing the Kremlin with strategic military bases.
Russian President Vladimir Putin visited the four islands Japan claims as its own for the first time on Thursday, prompting a "strong" protest from Tokyo.
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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 the Islands, with Russian media saying he visited a fish-processing plant where he was given caviar. Putin travelled to the islands after visiting the nearby Russian island of Sakhalin, where Russia's navy was conducting drills.
Japan, whose ties with Russia have worsened further since the full-scale invasion of Ukraine in 2022 – just as Moscow's relations with China and North Korea have warmed – objected strongly.
"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," said Foreign Minister Toshimitsu Motegi in a statement.
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.
In 2010 then-president Dmitry Medvedev became the first Russian leader to visit the disputed territory. Medvedev also went there several times subsequently as prime minister under Putin’s presidency.
The four contested islands -- Kunashiri, Habomai, Shikotan and Etorofu -- are part of a volcanic archipelago that runs in a broad arc from Russia's Kamchatka peninsula down to Japan's northernmost island, Hokkaido. The Soviet Union only declared war on Japan on August 8, 1945, after the atomic bomb was dropped on Hiroshima and six days before the end of the conflict.
The islands of some 20,000 residents experience harsh climates, but they 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.
On several occasions, off-and-on talks between Tokyo and Moscow to resolve the standoff appeared to be making progress. In 1956, when diplomatic relations were established, Soviet leader Nikita Khrushchev suggested returning two of the four disputed islands under a peace treaty.
Although talks resumed after the Soviet Union fell in 1991, no progress has been made, and the islands remain under Russian control.