Image copyright Reuters Image caption The US State Department currently warns US citizens not to travel to North Korea

The US is to ban its citizens from travelling to North Korea.

State Department spokeswoman Heather Nauert said in a statement that the ban would be published next week in the Federal Register, to come into effect 30 days later.

US officials linked the move to the death of jailed American student Otto Warmbier.

Once the ban is in effect, US citizens will need special validation to travel to or within North Korea.

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.

How did the news come to light?

Koryo Tours and Young Pioneer Tours, who both operate in North Korea, 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.

What form will the ban take?

Ms Nauert's statement said: "Due to mounting concerns over the serious risk of arrest and long-term detention under North Korea's system of law enforcement, the Secretary has authorised a Geographical Travel Restriction on all US nationals' use of a passport to travelling through, or to North Korea.

"Once in effect, US passports will be invalid for travel to, through, and in North Korea, and individuals will be required to obtain a passport with a special validation in order to travel to or within North Korea.

"We intend to publish a notice in the Federal Register next week.

"The restriction will be implemented 30 days after publication."

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".

How have the travel agencies reacted?

Simon Cockerell of Koryo Tours told the BBC the agency would still conduct tours and take Americans until the ban came into effect.

Media playback is unsupported on your device Media caption Otto Warmbier, fourth from right, seen shortly before his arrest

"If their country allows them to go, we will take them," he said.

Mr Cockerell 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.

Why now?

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.

Apart from the treatment of Americans in North Korea, tension has been increasing over Pyongyang's nuclear programme.

Media playback is unsupported on your device Media caption Take a tour of the demilitarised zone between North and South Korea

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.

How many Americans will it affect?

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.

What happened to Otto Warmbier?

Image copyright Reuters Image caption Otto Warmbier appeared in a news conference in 2016 confessing to stealing a propaganda sign

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".

Are there any Americans still being detained?

Yes. There are reported to be three US citizens in custody:

Kim Dong-chul, a 62-year-old naturalised US citizen born in South Korea, who was sentenced to 10 years of hard labour in April 2016 for spying

Korean-American professor Kim Sang-duk (or Tony Kim) who was detained in April 2017. The reasons for his arrest are not yet clear

Kim Hak-song, like Kim Sang-duk, worked at the Pyongyang University of Science and Technology (PUST) and was detained in May 2017 on suspicion of "hostile acts" against the state

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.