North Korea said on Sunday it has detained another American citizen for suspicion of acts against the state, which if confirmed would make him the fourth U.S. citizen to be held by the isolated country amid diplomatic tensions.

Kim Hak Song, who was detained on May 6, worked for the Pyongyang University of Science and Technology, the North’s KCNA news agency said. A third U.S. citizen, Kim Sang Dok, was detained in late April for hostile acts, according to the North’s official media.

(Reporting by Jack Kim and Ju-min Park, editing by Larry King)