WASHINGTON/SEOUL (Reuters) - The U.S. government on Friday said it will bar Americans from traveling to North Korea due to the risk of “long-term detention” in the country, where a U.S. student was jailed while on a tour last year and later died.

U.S. Secretary of State Rex Tillerson has authorized a “Geographical Travel Restriction” on Americans to forbid them from entering North Korea, spokeswoman Heather Nauert said.

“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,” Nauert said.

The move was due to “mounting concerns over the serious risk of arrest and long-term detention under North Korea’s system of law enforcement,” she said.

Otto Warmbier, a 22-year-old American was sentenced last year to 15 years hard labor in North Korea for trying to steal a propaganda sign while on a tourist visit.

He returned to the United States in a coma on June 13 after being released on humanitarian grounds and died June 19. The circumstances surrounding his death are not clear, including why he fell into a coma.

North Korea has said through its state media that Warmbier’s death was “a mystery” and dismissed accusations that he had died as a result of torture and beating in captivity.

North Korea is currently holding two Korean-American academics and a missionary, a Canadian pastor and three South Korean nationals who were doing missionary work. Japan says North Korea has also detained at least several dozen of its nationals.

It was not known how many Americans were currently in North Korea and the State Department said it was not its practice to give numbers of U.S. citizens living in or travelling to a particular country.

FILE PHOTO: The 105-storey Ryugyong Hotel, the highest building under construction in North Korea, is seen from inside another hotel's room in Pyongyang, North Korea May 7, 2016. REUTERS/Damir Sagolj/File Photo

U.S. officials say North Korea will become the only country in the world Americans are banned from visiting.

The department said it plans to publish a notice in the Federal Register next week, starting a 30-day clock before the restriction takes effect, Nauert said.

She said Americans who wanted to travel to North Korea “for certain limited humanitarian or other purposes” could apply for special passports to do so.

North Korea allows foreign tourists to visit but their travel is strictly limited.

Hundreds of Americans are among the roughly 4,000 to 5,000 Western tourists who visit North Korea each year, according to U.S. Representative Joe Wilson, a Republican from South Carolina.

This year, Wilson introduced a bill with Democratic Representative Adam Schiff to ban Americans from travelling to North Korea as tourists, following the detention of at least 17 U.S. citizens in the past decade.

Anthony Ruggiero, a senior fellow at the Foundation for Defense of Democracies and former U.S. Treasury official, said the State Department action was important as it would limit North Korea’s ability to use detained Americans as bargaining chips with Washington as it has in the past.

Tom Bodkin, managing director of the UK-based adventure travel company, Secret Compass, said the travel ban was “a bit of a shame.”

“Travel between different cultures breaks down the preconceptions that you have about different cultures and breaks down the stereotypes that you have,” he said.

Secret Compass has brought three Americans among the 19 people it took to North Korea since launching tours there last fall, he said.

U.S. Army veteran Brian Sayler, 40, who traveled to North Korea for six days in May, said he opposed the pending ban.

“We’re telling our own people, essentially, you can’t go where you want to go, I don’t really understand it,” said Sayler, a resident of West Pittston, Pennsylvania, who works as a police officer for the Department of Veterans Affairs.

North Korea’s growing nuclear and missile threat is perhaps the most serious security challenge confronting U.S. President Donald Trump, who has vowed to prevent North Korea from being able to hit the United States with a nuclear warhead.

North Korea this month test-launched an intercontinental ballistic missile that experts believe has the range to reach Alaska and Hawaii and perhaps the U.S. Pacific Northwest.