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BEIJING — Former co-workers of an American being held in North Korea said they are "surprised" by news of his detention.

Kim Sang-duk, who also goes by Tony Kim, worked at China's Yanbian University of Science and Technology (YUST), on the Chinese border with North Korea, for "many years," according to spokeswoman Choi Maehwa.

The university's president and faculty were "surprised" to hear Kim had been detained by authorities at the weekend, she told NBC News on Tuesday.

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Pyongyang University of Science and Technology in North Korea said in a statement that Kim had been working there for "several weeks" and was held on Saturday at Pyongyang Airport as he was preparing to leave the country.

His detention brings the total number of Americans held by the isolated country to three, and comes amid worsening tensions between North Korea and the United States.

One faculty member from the YUST School of Business, who did not want to named because of the sensitivity of the situation, said they had known Kim for 15 years and described him as a well-liked Christian who had a good reputation with teachers and students and enjoyed playing soccer, swimming and biking.

The former co-worker said Kim, in his 50s, had lived with his wife on campus for a decade-and-a-half before quitting in March, 2016. They also said the couple lived with two sons who had attended a local international school before moving to the U.S. to pursue a college education.

American professor Tony Kim, also known as Kim Sang-duk via Facebook

Details of Kim's situation are scarce but the Swedish Embassy in Pyongyang, which looks after consular affairs for the United States because there is no official diplomatic relationship between the reclusive country and Washington, said it was aware of a Korean-American citizen recently being detained.

"It is this embassy's understanding that the information that a U.S. citizen has been detained in the DPRK is correct," Martina Aberg, deputy head of mission for the Swedish Embassy in North Korea, confirmed to NBC News.

"We do not comment on individual consular cases that we are working on," Aberg added.

At least two other Americans are currently held in North Korea. Last year, Otto Warmbier, then a 21-year-old University of Virginia student from suburban Cincinnati, was sentenced to 15 years of hard labor in prison after he confessed to trying to steal a propaganda banner. Kim Dong Chul, who was born in South Korea but is also believed to have U.S. citizenship, is serving a sentence of 10 years for espionage.