North Korea detained a U.S. citizen this week teaching at a university in the capital of Pyongyang without justification, media reports have confirmed.

Kim Sang-duk, known in America as Tony Kim, was due to fly out of Pyongyang on Saturday before he was detained by authorities, making him the third U.S. citizen in North Korean custody.

A statement from the Pyongyang University of Science and Technology confirmed that Kim was taken into custody by airport authorities “after several weeks of service, teaching at PUST.”

Kim, who is in his late 50s, was also involved in several aid programs for North Koreans and had previously worked as a professor at the Yanbian University in China.

Other American detainees include University of Virginia student Otto Warmbier, who was sentenced to 15 years hard labor after he was caught tampering with political propaganda in a hotel lobby, and 62-year-old Kim Dong-chul, who was sentenced to 10 years for allegedly spying on the North Korean regime.

North Korea has previously released detainees Kenneth Bae and Matthew Todd Miller from custody, in what analysts believed were potential bargaining chips in any negotiations between the two countries. However, the North Korean government has since urged Bae, a Christian missionary, to “commit suicide.”

The incident comes amidst rising tensions between the U.S. and North Korea, with Donald Trump warning this month that he was sending “an armada” into the region to fend off any potential threat.

Meanwhile, last week, North Korean forces held a military parade to celebrate the 105th birthday of former leader Kim Il-sung, who remains the country’s “eternal leader.” However, reports from South Korea also indicated that the country’s planned missile launch, intended to be a show of strength, ended in failure.

Nevertheless, this week, the country threatened a “super-mighty preemptive strike” that would reduce America to “ashes” if they are attacked.

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