From the U.S. State Department:

The Department of State continues to warn U.S. citizens about travel to North Korea (Democratic People's Republic of Korea, or DPRK). The North Korean government will detain, prosecute, and sentence anyone who enters the DPRK without first having received explicit, official permission and an entry visa from its government. Travel by U.S. citizens to North Korea is not routine, and U.S. citizens crossing into North Korea, even accidentally, have been subject to arrest and long-term detention. Since January 2009, four U.S. citizens have been arrested for entering North Korea illegally. In 2010, a fifth U.S. citizen, who had a valid DPRK visa in his U.S. passport, was arrested inside North Korea on unspecified charges. This replaces the Travel Warning for North Korea dated November 3, 2011, to remind US citizens about the risks involved in traveling to the DPRK. ...

U.S. State Department Warning