The Trump administration will ban American citizens from traveling to North Korea, U.S. officials said Friday, following the death of university student Otto Warmbier who died in June after falling into a coma in a North Korean prison.

Secretary of State Rex Tillerson had decided to impose "geographic travel restriction" for North Korea, the officials said, which would make it illegal to use U.S. passports to enter the country. They said the restriction would go into effect 30 days after a notice is published in the Federal Register, but it was not immediately clear when that would be. There was no announcement in Friday's editions of the government publication.

The officials were not authorized to publicly discuss the decision before it is announced and spoke on condition of anonymity. Two tour operators that organize group trips to North Korea said they had already been informed of the decision.

It wasn't clear how many Americans the move will effect, as figures about how many Americans go to North Korea are difficult for even the U.S. government to obtain. 

The U.S. strongly warns Americans against traveling to North Korea, but has not until now prohibited it despite other sanctions targeting the country. Americans who venture there typically travel from China, where several tour groups market trips to adventure-seekers.

Barring Americans from stepping foot in North Korea marks the latest U.S. step to isolate the furtive, nuclear-armed nation, and protect U.S. citizens who may be allured by the prospect of traveling there. Nearly all Americans who have gone to North Korea have left without incident. But some have been seized and given draconian sentences for seemingly minor offenses.

The travel ban comes as the Trump administration searches for more effective ways to ramp up pressure on North Korea over its nuclear weapons program. 

Pyongyang's recent successful test of an intercontinental ballistic missile - the first by the North - has created even more urgency as the U.S. seeks to stop North Korea before it can master the complex process of putting a nuclear warhead atop a missile capable of hitting the United States.

President Donald Trump has expressed frustration that his initial strategy - enlisting China's help and influence to squeeze the North economically and diplomatically - has not yielded major results. 

Trump's administration is also considering other economic steps including "secondary sanctions" that could target companies and banks - mostly in China - that do even legitimate business with North Korea, officials said.

Under U.S. 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 United States travelers. 

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. But the U.S. doesn't currently prohibit its passports from being used to travel to any countries, even though financial restrictions limit U.S. travel to Cuba and elsewhere.

If a passport ban were placed on North Korea, an American who violated it could face a fine and up to 10 years in prison for a first offense.

Warmbier, who died after being medically evacuated in a coma from North Korea last month, 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.

Tillerson had been weighing a North Korea travel ban since late April, when American teacher Tony Kim was detained in Pyongyang, according to a senior State Department official. Those deliberations gained even more urgency after Warmbier's death. Lawmakers in Congress have also pushed their own, legislative solutions to try to ban travel to the North.

Simon Cockerell, Beijing-based general manager of the Koryo Group, one of the leading organizers of guided tours to North Korea, said the ban would affect 800-1,000 Americans who visit North Korea annually. 

Although Pyongyang does not publish exact figures, Americans are thought to account for a mere 1 percent of all foreign visitors. Westerners make up 5 percent of total visitors, Americans about 20 percent of the Western contingent, according to statistics.

Cockerell said the ban would likely 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 criticize North Korea for being isolationist and now we're helping isolate them," Cockerell said. "That's not what soft power is about."

AP