Japan on Saturday lodged a protest over Russian Prime Minister Dmitry Medvedev's visit to one of four disputed Pacific islands which have strained ties between the two countries since the end of World War Two.
The decades-old argument over the territory, claimed by both states, could set back Japanese Prime Minister Shinzo Abe's efforts to court resource-rich Russia and keep the door open to dialogue, despite the Ukraine crisis.
The islands are known in Russia as the Southern Kuriles and referred to as the Northern Territories in Japan. Russia seized them in the last days of World War Two and the dispute has kept the two countries from signing a formal peace treaty since.
After Medvedev visited Iturup island on Saturday, senior Japanese foreign ministry official Hajime Hayashi lodged the protest with the Russian ambassador to Japan, Evgeny Afanasiev, by telephone, the ministry said.
Japanese Foreign Minister Fumio Kishida will delay a visit to Russia planned for the end of August, the Nikkei business daily said, but did not elaborate.
Japan had been hoping for a visit this year by Russian President Vladimir Putin for summit talks with Abe, Kyodo news agency said.
While on the island, Medvedev said Japan's attitude would not stop more such visits.
"Our position is simple: We want to be friends with Japan, Japan is our neighbor. We have a good attitude towards Japan, but this shouldn't be linked in any way with the Kurile islands, which are part of the Russian Federation," he said.
"Therefore we have made visits, we are visiting and we will make visits to the Kuriles."
Medvedev emphasized economic development plans for a region potentially rich in oil and gas and invited foreign investors.
"If it will be our neighbors the Japanese that's not bad. If it will be Korean or Chinese friends, that's also not bad," he said. "Whoever comes first will get benefits."
Russia ordered a quicker buildup of military facilities in the disputed islands in June, following comments by Putin in April that he was ready to discuss the issue, while blaming Japan for a lack of dialogue.
On Saturday, Russia also published a government resolution affirming the country's claim to the seabed and natural resources in the central part of the Okhotsk Sea between the Kuriles and the Russian mainland, adding more than 50,000 square km (19,000 square miles) to Russia's territory.
The resolution said the claim had been approved by a United Nations commission last year and followed consultations with Japan.
Russia plans to at least double oil and gas flows to Asia in the next 20 years and Japan has been forced to resort to huge fuel imports to replace lost nuclear energy, after its reactors were shut down because of the 2011 Fukushima disaster.
