The US has strongly warned its citizens against travelling to North Korea following the arrest of a fourth US citizen there. The State Department said Americans in North Korea are at serious risk of arrest and long-term detention under its system of law enforcement. <br/><br/>US citizen, Kim Hak-Song, was detained for hostile acts. Korean Central News Agency reported that the detained US citizen had worked for the Pyongyang University of Science and Technology. At least 16 US citizens have been detained in North Korea in the past ten years.