American citizens will be banned from traveling to North Korea beginning at the end of August, the State Department announced Friday.

“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,” spokeswoman Heather Nauert said in a statement.

When the ban goes into effect, U.S. travelers will need to obtain “special validation” from the State Department to legally travel to North Korea. Validation will be granted for “humanitarian” and other unspecified reasons.

The State Department said it intends to publish a notice in the Federal Register next week. “The restriction will be implemented 30 days after publication of the Federal Register notice announcing the restriction,” the statement said.

Young Pioneer Tours, the company with which American student Otto Warmbier traveled to North Korea, released a statement via Twitter on Friday morning, saying the ban would come into effect within 30 days of July 27.

After the 30 day grace period any U.S. national that travels to North Korea will have their passport invalidated by their government,” the Young Pioneer statement said.

According to the Reuters news agency, another tour operator which runs trips into North Korea, Koryo Tours, also said they had been told about the looming ban, adding that they did not know how long it would last.