"North Korean security personnel may regard as espionage unauthorized or unescorted travel inside North Korea and unauthorized attempts to speak directly to North Korean citizens," a State Department advisory read.
The State Department warned the government may impose a sentence of hard labor or death for "knowingly or unknowingly" violating the law.
Rhetoric against the United States has grown more severe in the wake of North Korea's underground nuclear test in February. The test was described by the North Korean government as part of a campaign against the United States.
U.S. Defense Secretary Chuck Hagel said last week the Defense Department would add more ground-based interceptors to its West Coast defense to protect against intercontinental ballistic missile attack.
"The United States has missile defense systems in place to protect us from limited ICBM attacks, but North Korea in particular has recently made advances in its capabilities and is engaged in a series of irresponsible and reckless provocations," he said.