Story highlights At least two other US citizens are in North Korean custody

The detained American is a professor, report says

(CNN) North Korea detained a US citizen for unknown reasons as he was planning to fly out of Pyongyang International Airport on Saturday morning.

Kim Sang Duk, also known as Tony Kim, was teaching at Pyongyang University of Science and Technology, a statement from the school said.

The detention comes amid a buildup of tension on the Korean Peninsula, with a North Korean newspaper saying Sunday that Pyongyang was ready to sink a US aircraft carrier conducting drills in the region.

Kim is the third US citizen in North Korean custody. The school said he was detained by authorities at the airport "after several weeks of service, teaching at PUST."

American citizen Kim Sang Duk, also known as Tony Kim. His former colleague, who does not wish to be named, verified this photo from 2011 over the phone to CNN.

"We understand this detention is related to an investigation into matters not connected in any way with the work of PUST," the school said.

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