North Korea admitted in 2002 that it kidnapped 13 Japanese citizens in the 1970s and '80s.
They were apparently used to train North Korean spies in Japanese language and customs.
Pyongyang sent five of them home later that year, but insisted the rest were dead.
Japan though has demanded proof and says more of its citizens may have been taken.
It has also refused to provide energy and economic aid to North Korea or normalise relations unless progress is made on resolving the issue.
North Korea made a breakthrough agreement on nuclear disarmament on February 13, raising hopes for an end to a long-running standoff with the United States and regional powers.