"It's important to stop the flow of cash and prospective hostages into North Korea especially after the recent death of Otto Warmbier", said Evans J.R. Revere, a former State Department official who specializes in North Korea.
Warmbier was detained at Pyongyang's airport on January 2, 2016, near the end of his tour group's stay in North Korea.
It was revealed in June that Warmbier had been in a coma for several months.
How did the news come to light?
Another tour operator, Young Pioneer Tours, said in a tweet that it had also been informed of the ban, citing the same date. "Being a member of a group tour or using a tour guide will not prevent North Korean authorities from detaining or arresting you".
TASS does not have any official comments from the United States authorities.
At least three Americans remained in North Korean custody.
Rather, he said, the public should see the preparation and education much like the work being done to prepare the public for hurricanes and tsunamis, which pose a greater risk to the state.
Officials in Hawaii are launching a campaign to help residents plan for a nuclear missile attack from North Korea - much to the dismay of the state's tourism industry.
How have the travel agencies reacted?
The North has conducted its fourth and fifth nuclear tests and a quick succession of missile-related activities since the beginning of 2016, after leader Kim Jong Un had pledged to improve ties with the South in a New Year's address.
The China-based company later announced it would no longer take visitors from the U.S to the country.
It wasn't clear how many Americans the move will effect, as the number of Americans who travel to North Korea is unknown.
There has been movement towards a ban for a while in the United States, which increased with the Warmbier death.
Two of them were teachers at a private school while the third person worked in a special economic zone in North Korea.
Mingzheng has reportedly served as a money laundering front for North Korea's state-run Foreign Trade Bank, which has been targeted by past US sanctions.
North Korea's economy grew 3.9 percent past year, the fastest pace in more than a decade, South Korea's central bank said on July 21.
Reacting to it, the U.S. had then said that it was an ICBM with an estimated range of 5,500 kilometers and capable of hitting Alaska.
U.S. intelligence indicates that North Korea is possibly planning to launch its next missile underwater as Pyongyang's submarine has been found sailing some 100 kilometers out to sea in global waters.
How many Americans will it affect?
In all, about 5,000 Western tourists have visited North Korea each year.
Travellers wanting to visit the North must go with a tour company.
What happened to Otto Warmbier? He confessed to taking a propaganda banner from his hotel in Pyongyang, but it's unclear if that confession was coerced.