A civic group says that some 400-thousand North Koreans belonging to a state-run labor group work in conditions that amount to modern-day slavery.

The Seoul-based human rights group made the claim on Wednesday at a news conference in Seoul, releasing a report based on its interviews with 18 North Korean defectors, including four former workers or so-called Stormers.

The civic group said that North Korean authorities extort about 900 million U.S. dollars a year from North Korean residents.

According to the report, Stormers have to serve about ten years, much like military service, and are mobilized for state construction projects with almost no pay. It also revealed that recruitment even targeted middle school graduates, with students in poor physical condition and from bad family backgrounds usually forced to serve as Stormers.