Japan has protested a visit by Russian Prime Minister Dmitry Medvedev to one of four Pacific islands it also claims and which have strained relations between the two countries since the end of World War II.
Medvedev arrived on August 22 on Iturup, one of four islands that are located just off Russia's far eastern coast and north of Japan.
The islands, which are known in Russia as the Kuriles, and referred to as the Northern Territories in Japan, have been the source of a decades-old dispute between the two states after Russia seized them at the end of World War II.
Hajime Hayashi, the head of the Japanese foreign ministry's European division, called the Russian ambassador in Tokyo over the visit, saying it was "extremely regrettable."
Japan and Russia, driven by energy interests, had hoped to begin restoring ties. Japan seeks access to Russia's oil and natural gas supplies, while Russia intends to double oil and gas flows to Asia in the next 20 years.