North Korea detained a U.S. citizen on Saturday, according to Park Chan-mo, chancellor of the Pyongyang University of Science and Technology.

Tony Kim, who also uses his Korean name Kim Sang-duk, was taken into custody as he was preparing to leave Pyongyang International Airport, according to the Associated Press.

Martina Aberg, deputy chief of mission at the Swedish Embassy in Pyongyang, confirmed to CNN that he was being held, though no reason was given for the detention.

Kim is a Korean-American who had previously taught at the Yanbian University of Science and Technology, which is based out of China.

His detention follows the arrests of two other Americans since 2015, including Korean-American pastor Kim Dong Chul, who was sentenced to 10 years in prison for espionage, and 21-year-old University of Virginia student Otto Warmbier, who was sentenced to 15 years in prison after being accused of trying to steal a propaganda banner.

The detention comes as the Trump administration has taken a hard line against the country and a week after North Korea attempted to test a missile, shortly before U.S. Vice President Mike Pence visited the region.