North Korea said Sunday it has detained an American citizen for suspicion of acts against the state. If the charges are confirmed, it would make him the fourth U.S. citizen to be held by the country as tensions increase between the two countries.

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

"The relevant authority is currently carrying out a detailed investigation into the crime of Kim Hak-song," KCNA said in a statement.

Otto Warmbier, a University of Virginia student, was sentenced to 15 years of hard labor in 2016 for removing a political sign, CNN reported. Kim Dong Chul, the president of a company involved in international trade and hotel services, was arrested in 2015 and is serving 10 years on espionage charges, CNN reported.