WASHINGTON — American citizens will be barred by the U.S. from traveling to North Korea beginning next month following a prohibition on using U.S. passports to enter the country, the State Department said Friday.

Secretary of State Rex Tillerson decided to impose a “geographical travel restriction” on North Korea following the death last month of American university student Otto Warmbier, who fell into a coma while in North Korean custody. The ban also comes amid heightened U.S. concern about Pyongyang’s recent advancements in its nuclear weapons and ballistic missile programs.

Figures on how many Americans visit North Korea are difficult for even the U.S. government to obtain. But Simon Cockerell of the Koryo Group, one of the leading organizers of guided tours to the country, said 800 to 1,000 Americans go annually and will be affected.

State Department spokeswoman Heather Nauert said in a statement, “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 U.S. citizen nationals’ use of a passport to travel in, through or to North Korea.”

The restriction will take effect in late August, 30 days after it is published as a legal notice in the Federal Register.

Once it takes effect, Americans wanting to travel to North Korea may do so legally only with a “special validation passport,” which will be granted by the State Department on a case-by-case basis for “certain limited humanitarian or other purposes,” the statement said.

It did not elaborate on what “other purposes” the department would consider. Americans who violate the restriction could face a fine and up to 10 years in prison for a first offense.

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

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. Over the past decade, at least 16 U.S. citizens have been detained, officials say.

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 mounting a nuclear warhead capable of hitting the United States.

President Donald Trump has expressed frustration that his initial strategy — enlisting China’s influence to squeeze the North economically and diplomatically — has not yielded major results. Trump’s administration is 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 say.

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 U.S. travelers.

Since 1967, such bans have been imposed intermittently on countries including Algeria, Iraq, Lebanon, Libya, Sudan, Cuba and North Vietnam. 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.

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.

Two tour operators that organize group trips to North Korea said they had already been informed of the decision by officials from Sweden, which represents U.S. interests in North Korea because the two countries lack diplomatic relations.

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.

Cockerell, the tour organizer, said the ban 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.”

Story: Matthew Lee, Josh Lederman