North Korea has a third American citizen in custody, according to reports from South Korea on Sunday.
According to local news agency Yonhap, a Korean-American man in his 50s surnamed Kim was arrested last week while preparing to leave the reclusive state from Pyongyang International Airport.
Kim was identified as a former professor at Yanbian University of Science and Technology, a Chinese institution close to the border with North Korea.
Although it is not clear why he was arrested, a series of foreign missionaries have previously been targeted by Pyongyang as a threat to the country's authoritarian regime.
Kim is known to have been involved in North Korean aid work.
Two other Americans -- 22-year-old student Otto Warmbier and 62-year-old Kim Dong-chul -- were sentenced to serve 10 years and 15 years of hard labor respectively in 2016.
Foreign detainees are generally released early by the North, but Pyongyang may seek to use prisoners as leverage in negotiations amid tensions over its development of nuclear weapons.
While Washington advises Americans not to travel to North Korea, the U.S. has no official ties with Pyongyang and its consular affairs there are instead handled by the Swedish Embassy.