WASHINGTON -- US nationals will be barred from traveling to North Korea starting Sept. 1 unless they have a special reason, the State Department confirmed Wednesday.

In a notice in the Federal Register, the department said all US passports will be declared invalid for travel to, in, or through the North in the absence of special validation.

"The Department of State has determined that the serious risk to United States nationals of arrest and long-term detention represents imminent danger to the physical safety of United States nationals traveling to and within the Democratic People's Republic of Korea," it said.

The department announced the ban last month following the death of US college student Otto Warmbier in June.

The 22-year-old University of Virginia student returned from the North in a coma after being sentenced to 15 years of hard labor for stealing a political propaganda sign from a hotel in Pyongyang in January last year.

North Korean officials claimed Warmbier fell into a coma two months later due to botulism and a sleeping pill. But doctors in the US said he had severe brain damage.

The ban will be in effect for one year unless extended or revoked by the secretary of state. Exceptions can be granted to professional journalists, Red Cross representatives, and others with "compelling humanitarian" reasons or a travel request that is "in the national interest."

US nationals who are currently in North Korea should leave the country before Sept. 1, the department later said in a press release.

Three more Americans are still detained in the North, all of them Korean-Americans.