Russian Prime Minister Dmitry Medvedev visited the Kuril Islands, sparking protests from Japan, which claims the archipelago in the northwest Pacific.
Medvedev visited Iturup, one of four islands in a chain that lies near the far eastern coast of Russia and north of Japan, Russian media reported.
According to the government's website, Medvedev toured the island's new airport, seaport, fish processing plant and attended a youth forum where he met with young scientists and teachers.
He and forum participants took selfies and spread a large Russian flag on the ocean shore for a joint photo on the occasion of the Russian Flag Day, which was posted on the Russian Prime Minister's Instagram account.
Hajime Hayashi, head of the European department at the Japanese Ministry of Foreign Affairs, telephoned the Russian ambassador in Tokyo to express his protest over Medvedev's visit to the archipelago, which Japan calls the Northern Territories, AFP reported.
The visit "contradicts Japan's stance on the Northern Territories and offends the feelings of the Japanese people... It is highly regrettable," Hayashi said.
Soviet soldiers occupied the islands after Japan's surrender in World War II.
The seven-decade-long dispute has threatened trade relations and prevented Moscow and Tokyo from signing a formal post-war peace treaty.
The Kremlin and Japanese Prime Minister Shinzo Abe had hoped to begin mending relations to revive trade, with Japan seeking greater access to Russia's vast oil and gas reserves, the French agency said.
Medvedev visited the islands in 2012, and Russia held military exercises there in 2014.
Both of these events were the cause of Tokyo's protest.
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