A tour operator that runs organized trips into North Korea announced Friday that it had been told the United States would soon ban its citizens from traveling to the “Hermit Kingdom.”

Young Pioneer Tours, the company with which American student Otto Warmbier traveled to the DPRK, released a statement via Twitter which said that the ban would come into force within 30 days of July 27.

“After the 30 day grace period any US national that travels to North Korea will have their passport invalidated by their government,” the statement said.

According to the Reuters news agency, another tour operator which runs trips into North Korea, Koryo Tours, also said they had been told about the ban, adding that they did not know how long it would last.

Otto Warmbier, an American student at University of Virginia, was medically evacuated from imprisonment in North Korea after having been sentenced to 15 years of hard labor. He was accused by the North Korean government of trying to steal a propaganda banner and was convicted of subversion.