During the 1955-56 negotiations on restoring Russo-Japanese relations, the Soviet Union used Japanese prisoners of war as "political hostages," according to Japanese diplomatic documents declassified Tuesday.

Diplomatic experts say it is unusual for Japanese documents that clearly state the Soviet Union used POWs as leverage to be made public, and there is speculation that Tokyo released the documents for political reasons of its own. The release came the same day that the Japanese government abandoned hopes of concluding a decades-old territorial dispute with Russia by the end of the year.

Both countries had agreed "to strive to" solve a dispute over a group of Russian-held islets north of Hokkaido "by 2000."

The talks are now scheduled to resume next month.

The Soviet Union called for negotiations to restore relations in June 1955. When the talks began, Tokyo urged the Soviets to grant the Japanese prisoners an early release.

But the Soviets said a peace treaty was necessary first because the prisoners were under arrest as war criminals.

The negotiations deteriorated as the two countries also tangled over sovereignty of the islets.

The top secret diplomatic documents were dated January 1956, just before the negotiations were reopened. They stated that the Soviets were "apparently using (the prisoners) as political hostages," as the Soviets were only releasing them a few at a time and continued to detain the most prominent figures.

In September 1955, Soviet leader Nikita Khrushchev told a group of Japanese Diet members visiting the Soviet Union that the restoration of relations between Russia and Japan and the issue of the POWs' release would be best solved together. This indicated the Soviets were going to continue detaining the POWs until relations were restored.

Historians say the Soviet Red Army captured some 2 million Japanese soldiers and civilians in August 1945 from the then Japanese colonies of Manchuria in northeastern China, North Korea and the Kurile Islands. Of them, some 575,000 were interned in Soviet prisons, 473,000 returned home, 47,000 were sent back to Manchuria and other places and about 55,000 died, according to the Health and Welfare Ministry.

The talks were finally settled in October 1956 with then Prime Minister Ichiro Hatoyama's visit to the Soviet Union.

Hatoyama signed the Russo-Japan joint declaration on Oct. 19, 1956. It included repatriation of the detainees and the return of the Habomai group of islets and Shikotan Island off Hokkaido after the signing of a Russo-Japanese peace treaty.