North Korea has detained another U.S. citizen, a Korean American professor, bringing to three the number of Americans being held in Pyongyang.

The Swedish Embassy in Pyongyang, which represents U.S. interests there because the United States does not have diplomatic relations with North Korea, confirmed to The Washington Post that a U.S. national had been detained. In Washington, the State Department said it was aware of the report.

Media in South Korea identified the man as Kim Sang-duk, a former professor at the Yanbian University of Science and Technology (YUST) in the northeastern Chinese city of Yanji, near the border with North Korea.

Kim was arrested at Pyongyang’s international airport Friday as he was waiting to board a flight, South Korea’s Yonhap News Agency reported.

Kim had been teaching a class in international finance and management at the Pyongyang University of Science and Technology, a sister institution, for a month and was leaving the country with his wife when he was arrested, the specialist website NK News quoted the chancellor of PUST, Park Chan-mo, as saying.

[What it’s like to be an American held in North Korea]

North Korea has taken a slew of Americans hostage in recent years and used them as bargaining chips in negotiations with the United States.

It is holding two other Americans.

Otto Warmbier, a University of Virginia student who went on a tour in North Korea while on his way to a study-abroad program in Hong Kong, was detained for allegedly trying to steal a propaganda sign from a Pyongyang hotel on New Year’s Day last year.

He was convicted of subversion in March after a court found that he had committed a crime “pursuant to the U.S. government’s hostile policy” toward North Korea and was sentenced to 15 years of hard labor.

[North Korea sentences college student to 15 years of hard labor]

He has not been seen since March 2016, when he was convicted, and when Swedish diplomats were last allowed to meet with him.

Another American, former Virginia man Kim Dong-chul, was charged with spying last April and was sentenced to 10 years in prison. Kim, who is in his early 60s, was born in South Korea but became a U.S. citizen in 1987, although he is thought to have been living in northeastern China in recent years.

Previous American detainees have been released after a few months following visits from high-profile Americans, including former presidents Bill Clinton and Jimmy Carter.

But so far, the North Korean regime has not used Warmbier and Kim as leverage.

Read more:

North Korea sentences former Va. man to 10 years of hard labor

North Korea sentences U-Va. student to 15 years of hard labor in prison

U-Va. student held in North Korea ‘confesses’ to ‘severe’ crime

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