Tour groups that specialize in travel to North Korea said Friday that the Trump administration plans to bar Americans from visiting the country starting next month, in apparent retaliation for the detention of U.S. citizens there and the death of a young tourist who was held for nearly 18 months before being flown home in a coma.

Two tour companies, Koryo Tours and Young Pioneer Tours, said they were told the ban would be announced July 27 and would take effect a month later.

The State Department had no immediate response to the reports.

Otto Warmbier, a University of Virginia student from Ohio, used China-based Young Pioneer Tours to travel to North Korea in January 2016, only to be arrested on charges of attempting to steal a propaganda poster and sentenced to 15 years imprisonment with hard labor. He died June 19 at age 22, six days after being released in a coma and flown home to Cincinnati.

“We have just been informed that the US government will no longer be allowing US citizens to travel to the DPRK (North Korea),” Young Pioneer Tours said in a statement. “It is expected that the ban will come into force within 30 days of July 27th. After the 30 day grace period any US national that travels to North Korea will have their passport invalidated by their government.”

Koryo issued a similar statement, saying that after the ban takes effect, “it will no longer be legal for anyone travelling on a US passport to visit the DPRK as a tourist.” North Korea’s official name is the Democratic People’s Republic of Korea, or DPRK.

Young Pioneer Tours announced last month that it would no longer take Americans to North Korea, saying that Warmbier’s death prompted a reevaluation. “Now, the assessment of risk for Americans visiting North Korea has become too high,” the company said.

The U.S. government had discouraged Americans from traveling to North Korea but had not issued an outright ban.

The reported July 27 announcement date coincides with North Korea’s “Victory Day,” on which the isolated communist regime marks the 1953 Korean War armistice with elaborate pageantry.

Three U.S. citizens, all Korean Americans, are known to remain imprisoned in North Korea, although – unlike Warmbier – none of them entered the country as tourists. One is a businessman who formerly lived in Northern Virginia, and two had taught at Pyongyang University of Science and Technology, a school attended by the children of North Korea’s elite.

The longest-held American is Kim Dong-chul, a businessman who was working in a special economic zone in North Korea when he was arrested in October 2015 and accused of being a South Korean spy.

Kim Sang-duk, a 58-year-old U.S. citizen also known as Tony Kim, was teaching accounting at the university. The first American to be detained since President Donald Trump took office, he was arrested at the Pyongyang airport in April while trying to fly to China with his wife. He was accused of unspecified hostile criminal acts.

Similar vague charges were lodged against Kim Hak-song, another teacher at Pyongyang University of Science and Technology, who was arrested in May as he was about to leave North Korea.

As of May, the university employed about 40 Americans, most of them ethnic Koreans.

In recent years, tour companies have reported that roughly 800 to 1,200 Americans have visited North Korea annually.

In all, about 5,000 Western tourists have visited North Korea each year.