"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 today 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.
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."
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