The ban will be announced on July 27 and go into effect 30 days later, according to Koryo Tours, which bills itself on its website as experts in travel to unusual destinations, including North Korea. The Swedish embassy in Pyongyang, which handles consular affairs for the United States in the North, informed Koryo of the ban, but did not say how long it would last, the company said.
"But it does seem to be the case that in just over a month, it will be impossible for Americans to visit as tourists," Koryo Tours general manager Simon Cockrell told Reuters by telephone.
Another tour operator, Young Pioneer Tours, said on its website it had also been informed of the ban, citing the same date.
"After the 30-day grace period any U.S. national that travels to North Korea will have their passport invalidated by their government," it said.
A US official, who did not want to be identified, confirmed that the United States planned to announce a ban and said the State Department would provide details. The department had no immediate comment.
He was arrested there and sentenced to 15 years of hard labour for an incident at his hotel where he was accused of trying to steal a propaganda banner.
North Korea released the 22-year-old Warmbier in June in a coma and he died days after getting back to the United States. 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. [nL3N1JK355]
The isolated North Korea allows foreign tourists to visit but their travel is strictly limited.