US President Trump ban its nationals to travel to North Korea after Otto Warmbier's death.

American student Otto Warmbier died without recovering, having been brought back to the US last week. His family blames the North Korean authorities for his death.

The United States confirmed Friday that it plans to ban its nationals from travelling to North Korea, in the wake of the death of an American student who was imprisoned by Pyongyang during a tourist visit.

Travel agencies organizing trips to the isolated country had said earlier Friday they were informed of the impending change.

Strict warnings against travel to North Korea had already been in place, but Washington toughened its stance after the death in June of Otto Warmbier.

The 22-year-old University of Virginia student was imprisoned 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.

State Department spokeswoman Heather Nauert said the new restriction would be formally published in the government's Federal Register next week, and take effect one month later.

"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 US citizen nationals' use of a passport to travel in, through, or to North Korea," Nauert said.

"Once in effect, US passports will be invalid for travel to, through and in North Korea," she said.

Those seeking authorization for travel for "certain limited humanitarian or other purposes" would only be able to do so with a "special validation passport," the spokeswoman added.

"The safety and security of US citizens overseas is one of our highest priorities."

After Warmbier's 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."

-  Bad for tour business  -

China-based Young Pioneer Tours, which had taken Warmbier to North Korea, and Koryo Tours said the ban will be published on July 27 the anniversary of the end of the Korean War.

"We have just been informed that the US government will no longer be allowing US citizens to travel to the DPRK (North Korea)," Young Pioneer Tours said on its website.

Young Pioneer Tours had already said it would no longer take Americans to North Korea in the wake of Warmbier's death.

Koryo Tours general manager Simon Cockerell told AFP that his company had been notified by the Swedish embassy in Pyongyang, which usually acts on behalf of the United States in North Korea since Washington has no diplomatic ties with the isolated regime.

The official announcement "will basically end American tourism" in North Korea, said Cockerell, whose company currently takes between 300-400 Americans to the country each year.

While the decision will be bad for business, he said he sees it as more damaging to "North Koreans who are interested in having a balanced portrayal of what Americans are really like."

-  Bargaining chips  -

Legislation was introduced in the US House of Representatives in May that would have forbidden Americans from traveling to North Korea. That measure is still pending.

It cited the risk of their use as "bargaining chips in negotiations over a variety of issues," including Pyongyang's nuclear weapons program.

California Democrat Adam Schiff, who introduced the bill, welcomed the State Department's move, saying in the wake of Warmbier's death, "limiting US travel is unfortunately sensible and necessary."

Travellers wanting to visit the North must go with a tour company. They are required to fly to Pyongyang from Beijing, while other nationalities are allowed to go by train.

But the US State Department has strongly warned US citizens against travelling there.

Warmbier's death added to already high tensions in the region over North Korea's weapons ambitions, culminating in the country's successful test launch earlier this month of an intercontinental ballistic missile that experts say could reach Alaska.

North Korea has accused the United States of waging a "smear campaign" and denied that Warmbier was tortured or abused.

At least three Americans remain in North Korean custody: two professors, Kim Hak-Song and Kim Sang-Duk (or Tony Kim), who were both working at the Pyongyang University of Science and Technology (PUST); and Korean-American Kim Dong-Chul, who was sentenced to 10 years of hard labor for spying.