North Korea has arrested a Korean-American professor 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.

No reason was given for the arrest but it appears the professor has been caught in the web of a stand-off between the United States and North Korea over the latter’s continued romance with nuclear weapons.

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 U.S. aircraft carrier conducting drills in the region.

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

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

“We cannot comment on anything that Mr. Kim may be alleged to have done that is not related to his teaching work on the PUST campus.”

The statement said “life on campus and the teaching at PUST is continuing as normal” for the spring semester.

The detained American is a professor, the South Korea’s Yonhap News Agency reported.

The detention was also confirmed by Martina Aberg, deputy chief of mission at the Swedish Embassy in Pyongyang.

The embassy represents U.S. interests in North Korea, since Washington and Pyongyang do not have direct diplomatic relations.

“He was prevented from getting on the flight out of Pyongyang,” Aberg said. “We don’t comment further than this.”

The U.S. State Department on Sunday said it was working on the case.

“We are aware of reports that a U.S. citizen was detained in North Korea,” said a department official. “The protection of U.S. citizens is one of the Department’s highest priorities.”

The official said the United States would work with the Swedish Embassy on the matter, but did not provide further comment.

At least two other U.S. citizens are known to be in North Korean custody.

Otto Warmbier, 21, a student at the University of Virginia, was detained at Pyongyang airport on January 2 last year after visiting the country with a tour group.

He has since been sentenced to 15 years of hard labour for allegedly removing a political sign from a hotel wall.

Kim Dong Chul, a naturalized U.S. citizen of Korean origin, was arrested on October 2015.

Last year, North Korea sentenced him to 10 years of hard labour on espionage charges.