North Korea said on Sunday it has detained another U.S. citizen on suspicion of “hostile acts” against the state, which would make him the fourth American to be held by the isolated country amid heightened diplomatic tensions with Washington. Kim Hak Song, who worked for the Pyongyang University of Science and Technology, was detained on Saturday, the North’s KCNA news agency said. “A relevant institution of the DPRK detained American citizen Kim Hak Song on May 6 under a law of the DPRK on suspension of his hostile acts against it”, KCNA said. DPRK is short for the Democratic People’s Republic of Korea, North Korea’s official name. A third U.S. citizen, Kim Sang Dok, who was associated with the same school, was detained in late April for hostile acts, according to the North’s official media.