A Japanese man was taken into custody in North Korea earlier this month for a yet unknown reason, and Tokyo is seeking his release, Japanese government sources said Saturday.

The man is in his 30s and entered North Korea as a member of a tour organized by a foreign travel company, the sources said. The man, who apparently had visited North Korea before, was detained while in the western port city of Nampo, they added.

A Foreign Ministry official would not say whether the ministry knew his purpose in visiting North Korea.

North Korean Foreign Minister Ri Yong Ho also would not respond to reporters' questions about the matter when he arrived at Beijing international airport Saturday following a trip to Singapore and Iran.

The Japanese government has asked its citizens to refrain from traveling to North Korea as part of economic sanctions imposed on the country to compel Pyongyang to abandon its development of nuclear weapons and offensive missiles.

In 1999, a Japanese newspaper reporter was taken into custody in North Korea on spy charges and detained for about two years.

The current incident may complicate Japan's efforts to organize a meeting between Prime Minister Shinzo Abe and North Korean leader Kim Jong Un in a bid to resolve issues concerning the abduction of Japanese by North Korean agents in the 1970s and 1980s.

Japan officially lists 17 citizens as having been abducted but suspects North Korea's involvement in even more disappearances. The abduction issue remains an obstacle to the normalization of diplomatic ties between the countries.

The Japanese government has been seeking to organize an Abe-Kim summit since U.S. President Donald Trump decided in March to meet Kim.