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7 year oldKoryo Tours and Young Pioneer Tours said the ban would be announced on 27 July to come into effect 30 days later.
They were informed by the Swedish embassy, which conducts US affairs in the country.
The US has not yet confirmed the ban but there has been momentum, given the worsening ties and the death of jailed American student Otto Warmbier.
Mr Warmbier travelled to North Korea with Young Pioneer Tours. He was arrested in 2016 for trying to steal a propaganda sign and sentenced to 15 years in prison. He was returned to the US in a coma in June and died a week later.
Koryo Tours and Young Pioneer Tours both revealed on Friday that they had been told of the upcoming ban by the Swedish embassy, which acts for the US as Washington has no diplomatic relations with Pyongyang.
Rowan Beard, of Young Pioneer Tours, told the BBC the embassy was urging all US nationals to depart immediately.
He said the embassy was trying to check on the number of US tourists left in the country.
There has been no confirmation from the US. The state department continues to have an alert dated 9 May strongly warning US citizens not to travel to North Korea.
A Young Pioneer Tours statement said: "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."
Rowan Beard said that the 30-day grace period would "give leeway for any [Americans] currently in the country as tourists or on humanitarian work".
Simon Cockerill, of Koryo Tours, said: "It remains to be seen what the exact text is, but the indication is it's just a straight up ban on Americans going."
Mr Cockerill told the BBC the agency would still conduct tours and take Americans until the ban came into effect.
"If their country allows them to go, we will take them," he said.
Mr Cockerill added: "It's unfortunate for the industry but also for North Koreans who want to know what Americans are really like."
After the death of Mr Warmbier, the China-based Young Pioneer Tours announced it would no longer take visitors from the US to the country.
There has been movement towards a ban for a while in the US, which increased with the Warmbier death.
In May, two congressmen introduced the North Korea Travel Control bill to cut off the foreign currency the country earns from American tourists.
The House foreign affairs subcommittee is scheduled to take up the draft legislation on 27 July but it would still have to go to the Senate. So there could be an executive order.
Last month, Secretary of State Rex Tillerson said: "We have been evaluating whether we should put some type of travel visa restriction to North Korea. We have not come to a final conclusion, but we are considering it."
Apart from the treatment of Americans in North Korea, tension has been increasing over Pyongyang's nuclear programme.
This month North Korea announced it had successfully tested what it said was its first intercontinental ballistic missile (ICBM), the latest in a series of tests in defiance of a UN ban.
Its range has been disputed, but some experts said it could reach Alaska.
The US and South Korea then conducted a ballistic missile drill and issued a stark warning to the North.
Some are suggesting the US is using the date the ban is set to be announced - 27 July - to cloud North Korea's Victory Day on the same day.
North Korea only relaxed its rules for American visitors in 2010.
The state department does not keep a record of the number of American tourists.
Tour operators suggest that up to 1,000 visit every year.
Otto Warmbier, 22, was an economics student who was arrested on 2 January 2016 and confessed to trying to take a propaganda sign from a hotel.
He was sentenced to 15 years of hard labour.
In June, North Korea said he had been in a coma for a year after contracting botulism.
He was flown back to the US on 13 June but died a week later without regaining consciousness.
His family rejected North Korea's version of events, saying he had been subjected to "awful torturous mistreatment".
Yes. There are reported to be three US citizens in custody:
The US has in the past accused North Korea of detaining its citizens to use them as pawns in negotiations over its nuclear weapons programme.
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