On Sunday, the North's official Korean Central News Agency said the country has detained a Korean-American man, identified as Kim Hak-song, on suspicion of unspecified "hostile acts." His detention raised the number of US citizens held in the reclusive state to four.

KCNA said Kim had worked for Pyongyang University of Science and Technology.

Last week, the North announced that another American citizen, Kim Sang-dok, who taught at the same university, was arrested on April 22 for "committing criminal acts of hostility aimed to overturn" the country.

The other detainees are college student Otto Warmbier and Kim Dong-chul, a 63-year-old Korean-American.

Warmbier was sentenced to 15 years of hard labor for trying to steal a propaganda banner from a Pyongyang hotel, while Kim was sentenced to 10 years at hard labor on charges of espionage and subversion.

American visitors have often been detained in North Korea on charges of anti-state and other unspecified crimes. Widespread views are that the communist nation has used the detentions as bargaining chips in its negotiations with Washington.

Kenneth Bae, a Korean-American missionary, served two years of detention in the North before being released in November 2014 when US Director of National Intelligence James Clapper made a secret trip to Pyongyang to win his release and that of another US detainee. (Yonhap)