Japan's Chief Cabinet Secretary, Yukio Edano, plans to tour the four disputed North Pacific islands by plane on Saturday.

His move is a reaction to visits by Russian officials to the disputed territory. Japan is also urging companies from Asian countries not to invest in Russian exploitation of the islands.

Russian President Dmitry Medvedev provoked a storm of protest from Tokyo in November by making the first visit by a Kremlin leader to the southern Kuril islands -- known by Japan as the Northern Territories.

Russia and Japan both claim sovereignty over the four North Pacific islands. The dispute has blocked a peace treaty between the two countries since the end of World War Two.

Russia's Foreign Ministry says the long sought peace treaty must not only cover the territorial dispute, but also mark a new level of cooperation in politics, trade, the economy, innovation and technology.