A US citizen has reportedly been arrested as he tried to fly out of North Korea, becoming the third American to be detained there. <br/><br/>South Korea's Yonhap news agency said this today. The US State Department said it was aware of the reports, but there was no official confirmation of the arrest. <br/>'<br/>Yonhap quoted sources as saying the man, identified only by his surname Kim, was arrested on Friday at Pyongyang<br/>International Airport on his way out of the country. <br/><br/>It said Kim, aged in his late 50s and a former professor at China's Yanbian University of Science and Technology, had been involved in aid programmes for the North.'<br/><br/>He reportedly was in North Korea for about a month to discuss relief activities. The reason for his arrest was unclear.'<br/>'<br/>Two other US citizens college student Otto Warmbier and Korean-American pastor Kim Dong-Chul are currently being held in the North after being sentenced to long prison terms.'