TOKYO (Jiji Press) -- Japanese Prime Minister Sanae Takaichi on Thursday condemned Russian President Vladimir Putin's visit to one of the four northwestern Pacific islands claimed by Tokyo but controlled by Moscow.

Putin's visit to Etorofu, one of the islands Japan calls the Northern Territories, is "incompatible with Japan's consistent position and is absolutely unacceptable," Takaichi told reporters at her official residence.

She said Tokyo "strongly protests" the trip, adding, "The Northern Territories are an inherent part of our country's territory both historically and under international law."

The prime minister discussed possible responses with officials including Keiichi Ichikawa, secretary-general of the National Security Secretariat, and Vice Foreign Minister Takehiro Funakoshi.

Later in the day, Foreign Minister Toshimitsu Motegi summoned Russian Ambassador to Japan Nikolay Nozdrev to the Foreign Ministry and lodged a protest.

Takaichi told reporters that she learned of Putin's visit to Etorofu on Thursday afternoon. The visit came as a surprise, a Japanese government source said. Russia's state-run news agency Tass reported that the president visited the island Thursday.

"This visit has further hardened Japanese public sentiment against Russia and made it difficult to improve bilateral relations in the medium to long term," she said. "We want the Russian side to take the gravity of this matter seriously."

The Russian-controlled islands were seized by the former Soviet Union from Japan at the end of World War II and have long been claimed by Japan. The territorial row has prevented Tokyo and Moscow from concluding a peace treaty to formally end their wartime hostilities.

The Takaichi administration has continued to impose sanctions against Russia following Moscow's invasion of Ukraine, which started in February 2022, in step with other Group of Seven major democracies. It has been increasing vigilance over Russian military exercises in the waters around the Northern Territories and deeper military cooperation between Russia and China.

At the same time, Tokyo has pursued diplomacy with Moscow amid energy security concerns stemming from the turmoil over Iran, as well as concerns about the potential impact of the Middle East situation on the Northern Territories issue.

As part of such efforts, senior officials from the Ministry of Economy, Trade and Industry visited Russia in May. Motegi chatted with Russian Foreign Minister Sergei Lavrov at a foreign ministers meeting linked to the Association of Southeast Asian Nations in the Philippines last month.

Former residents of Etorofu island expressed outrage over Putin's visit. Yuzo Matsumoto, 85, who was born on the island and lived there until the age of 4, said he feels as though his "hometown has been trampled on."

Matsumoto, who heads a group of former residents of the Northern Territories, said he is "extremely frustrated and disappointed."

According to the group, the number of former residents of the Northern Territories stood at about 4,700 as of June, with their average age topping 90.

"It is deeply upsetting to former residents who were forced out," said Takako Yamashita, 62, who engages in advocacy work regarding the disputed islands with her mother, who is from Etorofu. "It's frustrating to be unable to do anything."