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). 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 arbitrary arrest and long-term detention. Since January 2009, four U.S. citizens have been arrested for entering North Korea illegally, and two U.S. citizens who entered on valid DPRK visas were arrested inside North Korea on other charges. ...
U.S. citizens should also consult the Department of State's Country Specific Information for North Korea and the current Worldwide Caution, which are located on the Department's travel website at travel.state.gov. U.S. citizens can obtain current information on safety and security conditions by calling 1-888-407-4747 toll-free in the United States and Canada or, from outside the United States and Canada, 1-202-501-4444. These numbers are available from 8:00 a.m. to 8:00 p.m. Eastern Time, Monday through Friday (except U.S. federal holidays).
Stay up to date by bookmarking our Bureau of Consular Affairs website, which contains the current Travel Warnings and Travel Alertsas well the Worldwide Caution.
U.S. State Department Warning