Japan's Prime Minister Sanae Takaichi has criticised Vladimir Putin's first-ever visit to the disputed Kuril islands on Thursday as "absolutely unacceptable".

Russian state media reported that Putin visited Iturup, known in Japan as Etorofu, one of four islands in the Kuril chain that are claimed by Russia and Japan.

Putin's trip "has worsened Japanese sentiment towards Russia," Takaichi told reporters, maintaining the island chain was "a unique territory of [Japan] historically and under international law".

The countries have been locked in a dispute over the Kurils for decades, and it is the main reason Moscow and Tokyo did not sign a peace treaty after World War Two.

The Kuril islands sit between Hokkaido in northern Japan and Russia's Kamchatka Peninsula. Four of the southernmost islands are claimed by Japan, which calls them the Northern Territories.

They are administered by Russia. At the end of World War Two, the Soviet Union annexed the island chain and deported around 17,000 Japanese residents.

During his visit, Putin sampled fish roe and spoke to island locals, Russian state media reported.

Earlier on Thursday,Japan's foreign minister Toshimitsu Motegi said the islands were "inherently Japanese territory both historically and under international law, and the Government of Japan strongly protests this visit".

Over the years, attempts to resolve the territorial dispute have made little progress. Bilateral ties have also remained frosty, with Japan imposing sanctions against Russia for its invasion of Ukraine in 2022.

Japan has protested against previous visits by Russian officials to the disputed islands, including Russian Prime Minister Mikhail Mishustin and his predecessor Dmitry Medvedev.

The Kuril islands are a strategic area for Russia, which has beefed up its military presence on the islands over the past decade. The island of Iturup is used as its primary military air base in the region.

Last month, Chinese and Russian naval warships sailed through a strait in the Kuril islands during a joint patrol.

Russia also holds regular military drills near the islands, drawing condemnation from Japan.