South Korea-based Yonhap reported that the US citizen, identified only by his surname Kim, was arrested by North Korean authorities on Friday at Pyongyang International Airport on his way out of the country.

The sources said Kim, a former professor at the Yanbian University of Science and Technology in his late 50s, has been engaged in aid and relief programs to North Korea.

He reportedly was in the North for about a month to discuss relief activities. The reason for his arrest was not clear, Yonhap online reported.

The detainment is the latest in a series of American nationals being arrested in North Korea. Two other U.S. citizens — college student Otto Warmbier and Korean-American pastor Kim Dong-chul — are also currently detained in the North after being sentenced to long prison terms for what Pyongyang calls subversive acts against the country.

Ahn Chan-il, director of the World North Korea Research Center in Seoul, said that the North “seems to be intending to use professor Kim as leverage in negotiations” amid the current bad relations between the two countries.

According to Yonhap, the National Intelligence Service, South Korea’s spy agency, said it “was not aware” of Kim’s arrest.

Since 2009, over 10 U.S. citizens have been detained in North Korea on charges of anti-state and other unspecified crimes. The widespread view has been that Pyongyang uses the detentions as bargaining chips in its negotiations with Washington.