Russian Prime Minister Dmitry Medvedev on Friday made his first visit since 2015 to one of the four Russian-held islands off Hokkaido that are claimed by Japan, despite a request from Tokyo to have the trip canceled, according to Russia’s Interfax news agency.

Medvedev traveled to Etorofu, which is called Iturup in Russia. The island is part of what Russia calls the Southern Kurils, which are known as the Northern Territories in Japan. Tokyo claims they were illegally seized by the Soviet Union following Japan’s surrender at the end of World War II.

It was Medvedev’s fourth visit to one of the disputed islands.

Japan lodged a strong protest against Medvedev’s visit to Etorofu through diplomatic channels.

But Medvedev claimed that the islands are Russia’s territory, suggesting that Moscow will not accept the Japanese protest, according to Russian news agency Tass.

He was scheduled to visit a seafood processing factory and a hot-spring facility, check on progress in improving infrastructure such as roads and schools, and go fishing on a yacht, according to Russian government officials.

With bilateral negotiations on the dispute at a standstill, Medvedev’s visit is believed to be intended to highlight Moscow’s control of the island through its efforts to develop the remote territory.

Prime Minister Shinzo Abe plans to meet Russian President Vladimir Putin in the Russian city of Vladivostok next month on the sidelines of an economic conference.

But Medvedev’s trip may affect plans for joint economic programs on the disputed islands between the two countries.

Japan has been promoting joint development of the islands in its bid to secure their eventual return.

In November 2010, when he was president, Medvedev visited Kunashiri, another of the four islands, becoming the first Russian leader to visit one of the disputed isles.

He visited Kunashiri again in July 2012 as prime minister and Etorofu in August 2015.

Each visit has prompted a formal protest from Japan, and bilateral relations were particularly strained after he told Kunashiri residents in 2012 that Moscow would never return the islands.

The territorial dispute has prevented the two countries from concluding a postwar peace treaty.

Last November, Abe and Putin agreed to step up talks on a peace treaty based on a 1956 joint declaration that mentions handing back two of the islands — Shikotan and the Habomai islet group — to Japan.

But further progress has yet to materialize, with Putin apparently concerned that his falling support rate may drop further if he hands over the territory.