The Trump administration is to ban US citizens from travelling to North Korea following the death of university student Otto Warmbier who fell into a coma in a North Korean prison.
US officials said that Secretary of State Rex Tillerson has decided to implement a "geographical travel restriction" for North Korea, which would make the use of US passports to enter the country illegal.
They said the restriction would be published in the Federal Register next week and will take effect 30 days after that.
Two tour operators that organise group trips to North Korea said they had already been informed of the decision.
Under US law, the secretary of state has the authority to designate passports as restricted for travel to countries with which the United States is at war, when armed hostilities are in progress, or when there is imminent danger to the public health or physical security of US travellers.
Geographic travel restrictions are rare but have been used by numerous administrations in the past for countries where it has been determined to be unsafe.
Since 1967, such bans have been imposed intermittently on countries such as Algeria, Iraq, Lebanon, Libya, Sudan, Cuba and North Vietnam.
In this case, the administration had been considering the step since Mr Warmbier died after being medically evacuated in a coma from North Korea last month.
He suffered a severe neurological injury from an unknown cause while in custody.
Relatives said they were told the 22-year-old University of Virginia student had been in a coma since shortly after he was sentenced to 15 years of hard labor in North Korea in March 2016. He had been accused of stealing a propaganda poster while on a tour of the country.
The United States, South Korea and others often accuse North Korea of using foreign detainees to wrest diplomatic concessions. At least three other Americans remain in custody in the North.
Simon Cockerell, Beijing-based general manager of the Koryo Group, one of the leading organisers of guided tours to North Korea, said the ban would affect 800 to 1,000 Americans who visit North Korea annually.
Although Pyongyang does not publish exact figures, Americans are thought to account for 1% of all foreign visitors. Westerners make up 5% of total visitors, and Americans about 20% of the Western contingent, according to statistics.
Mr Cockerell said the ban would probably have a tangible impact on business for his and similar outfits, and said that would turn back the clock on engagement with the North.
"It's unfortunate because we criticise North Korea for being isolationist and now we're helping isolate them," Mr Cockerell said. "That's not what soft power is about."
State Department spokeswoman Heather Nauert said Americans who seek to travel to North Korea will need a "special validation passport" after the ban takes effect.
She said those validations may be granted for "certain limited humanitarian or other purposes". All other travel by Americans "to, through and in" North Korea will be prohibited.
Ms Nauert said the ban is being put in place because of "mounting concerns" about "the serious risk of arrest and long-term detention under North Korea's system of law enforcement".