Japan has lodged a strong protest after Russian President Vladimir Putin visited a disputed island in the Kuril chain, describing the move as “absolutely unacceptable” and warning it could further damage relations between Tokyo and Moscow.

Putin visited Iturup, known as Etorofu in Japan, on 13 August in his first visit to the disputed islands. He toured a school, hospital and fish-processing facility and met local officials. Russia controls the islands, while Japan claims the southern part of the chain as its Northern Territories.

Japanese Prime Minister Sanae Takaichi said the visit was incompatible with Japan’s position that the islands are Japanese territory under its historical and legal claims. Foreign Minister Toshimitsu Motegi also summoned Russia’s ambassador to formally protest the visit.

Moscow has rejected Tokyo’s position, with Russian officials reaffirming that the islands are Russian territory. The dispute dates back to the final days of the Second World War, when Soviet forces seized the islands. It has remained a major obstacle to a formal peace treaty between Japan and Russia.

The dispute is another sign of the increasingly complex security environment across the wider Pacific region. Japan has become an increasingly active partner for Pacific island countries, including Tonga, where Tokyo’s engagement includes climate, development and security cooperation.

Japan’s defence authorities have also identified Russian military activity in the Far East and growing military cooperation between Russia and China as security concerns affecting the wider Indo-Pacific.

The immediate question is whether Tokyo’s protest will lead to additional measures against Moscow, with Japan already considering further sanctions.