Diplomatic tensions have risen sharply between Russia and Japan following a visit by President Dmitry Medvedev to disputed islands in the Pacific Ocean.
Moscow and Tokyo have summoned each other's ambassadors to explain their positions.
The BBC reports Mr Medvedev met residents in Kunashir, the second largest of the four islands, and pledged more investment for the region. The island is known in Japan as Kunashiri.
Russia took control of the islands at the end of World War II. They lie to the north of Japan's Hokkaido island and to the south of Russia's Kamchatka peninsula.
They are known in Russia as the Southern Kurils, while Japan calls them the Northern Territories.
Mr Medvedev is the first Russian leader to set foot on the islands, which have rich fishing grounds, mineral deposits and possibly oil and gas reserves.
The BBC reports the dispute has strained relations between Tokyo and Moscow ever since World War II, preventing the signing of a formal peace treaty. Both nations claim the islands are "our lands".
Japan is also involved in a territorial dispute with China over islands they both claim in the South China Sea.