The government will demand that Russia honor a 2001 agreement to seek the conclusion of a World War II peace treaty by resolving the ownership dispute over all four Russian-held islands off Hokkaido claimed by Japan, not just a portion of them, sources said Monday.

The Abe administration wants to ensure that it and Moscow are on the same page on the issue before it invites President Vladimir Putin to visit Japan this year, they said.

Japan and Russia agreed at a summit between then-Prime Minister Yoshiro Mori and Putin in 2001 in Irkutsk that they would work toward signing a peace treaty by resolving the dispute over the sovereignty of the four islands.

The administration is concerned because Russia has been strengthening its control of the islands by building new infrastructure and expanding military drills.

The islands — called the Northern Territories in Japan and the Southern Kurils in Russia — were seized by the Soviet Union following Japan's surrender in World War II on Aug. 15, 1945.

The dispute has prevented the two countries from concluding a World War II peace treaty.

Japan is seeking recognition of its ownership of the islands — Kunashiri, Etorofu, Shikotan and the Habomai group of islets — while taking the position it would be flexible about how and when they are returned.

Russia has taken a position in line with the Soviet Union's 1956 joint statement with Japan that it agrees to return Shikotan and Habomai following the conclusion of a peace treaty.