The U.S. State Department on Friday said U.S. citizens would be subject to a geographic travel restriction, barring them from visiting North Korea, citing the risk of long-term imprisonment.

The news follows similar reports from Friday, when several travel companies which conduct tours to the DPRK said the ban would be in effect from July 27.

“Due to mounting concerns over the serious risk of arrest and long-term detention under North Korea’s system of law enforcement, the Secretary has authorized a Geographical Travel Restriction on all U.S. citizen nationals’ use of a passport to travel in, through, or to North Korea,” State Department Spokesperson Heather Nauert said during a briefing on Friday.

“Once in effect, U.S. passports will be invalid for travel to, through and in North Korea, and individuals will be required to obtain a passport with a special validation in order to travel to or within North Korea.”

Further details on the ban will be issued next week when the State Department publishes a notice in the Federal Register, the official journal of the United States government. The restriction will come into force 30 days from the publication date, Nauert added.

But the ban has scope to allow for U.S. citizens to visit the DPRK for non-touristic reasons, with further information to be made available on travel.state.gov.

“Individuals seeking to travel to North Korea for certain limited humanitarian or other purposes may apply to the Department of State for a special validation passport,” Nauert continued.

Neither the State Department or its Consular Affairs section would elaborate on the current legal basis for the ban, how long it would last, or if it could be circumvented by U.S. citizens with dual citizenship.

“I think they will use this authority, (but it’s) unclear how it will be implemented,” Anthony Ruggiero, Senior Fellow at the Foundation for Defense of Democracies said, citing Chapter 4, Title 22 of the United States Code of Laws.

“The statutory authority State is using here is a limitation on the use of a U.S. passport, so I don’t know how we could (a) prevent a dual national from using a non-U.S. passport for travel to North Korea, or (b) expect the North Koreans to cooperate with this restriction by stamping U.S. passports,” Joshua Stanton, an attorney and author of the One Free Korea blog added.

The State Department’s ban follows announcements from some tour companies saying they would longer take U.S. citizens to the DPRK following the death of Otto Warmbier.

The 22-year old U.S. national was arrested in the DPRK last year and was returned last month in a coma, though tragically died a few days after his return.

It is thought approximately 1,250 U.S. tourists visit North Korea each year, a figure that would generate about $1.9 million dollars of U.S. consumer expenditure, a figure inclusive of fees towards travel, food, and accommodations.