North Korea has arrested another U.S. citizen for alleged anti-state crimes.

Kim Hak-song, a professor working at Pyongyang University of Science and Technology, was detained Saturday, according to reports from North Korea’s state-run Korean Central News Agency.

Kim was reportedly detained for “hostile acts” against the reclusive regime.

North Korea is currently holding four U.S. citizens, likely as hostages to be used as pawns in future negotiations, as has been the case in the past.

Kim Sang-duk (Tony Kim), also a professor at PUST, was detained April 22 for alleged efforts to harm the North Korean state and bring down the regime.

Other prisoners include Otto Frederick Warmbier, a former business student at the University of Virginia, and Kim Dong-chul, a businessman. Pyongyang detained the former for stealing a political poster. The latter was arrested on charges of espionage. Warmbier was sentenced to 15 years of hard labor, and Kim was sentenced to 10 years of labor.

As the U.S. has no formal diplomatic relations with North Korea, securing their release is extremely difficult.

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