Washington, United States (AFP) — The United States on Wednesday officially set a September 1 start date for a ban prohibiting its citizens from traveling to North Korea.

“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 (DPRK),” read the restriction as it appeared Wednesday in the US government’s Federal Register.

“All United States passports are declared invalid for travel to, in, or through the DPRK unless specially validated for such travel,” it added.

An exemption was noted for approved humanitarian travel and for journalists in some circumstances. The ban is to remain in effect for one year, unless it is revoked sooner by US Secretary of State Rex Tillerson.

The travel ban was first announced last month in the wake of the death of American student Otto Warmbier who fell into a coma after being imprisoned by Pyongyang during a tourist visit.

Warmbier, 22, a student at the University of Virginia, died in June after being held for more than a year on charges of stealing a propaganda poster from a North Korean hotel — and sent home in a mysterious coma that proved fatal.

After his death, President Donald Trump said he was determined to “prevent such tragedies from befalling innocent people at the hands of regimes that do not respect the rule of law or basic human decency.”

Warmbier’s death added to already high tensions in the region over North Korea’s weapons ambitions, culminating in two successful test launches by Pyongyang in recent weeks of an intercontinental ballistic missile that experts say could reach US territory.