The United States will introduce a ban on Americans visiting North Korea, tourist agencies have said.

Koryo Tours said the ban would be announced on 27 July and would go into effect 30 days later. It did not say how long the ban would last.

Another tour operator, Young Pioneer Tours, said in a Twitter message that it had also been informed of the ban, citing the same date.

Young Pioneer was the agency that took US student Otto Warmbier to North Korea. He was subsequently arrested there and sentenced to 15 years of hard labour.

North Korea released him in June in a coma and he died days after getting back to the United States.

China-based Young Pioneer said: “After the 30 day grace period any US national that travels to North Korea will have their passport invalidated by their government.”

North Korea has said through its state media that Warmbier’s death was “a mystery” and dismissed accusations that he had died as a result of torture and beating in captivity.

The isolated state allows foreign tourists to visit but their travel is strictly limited.

Efforts to exert pressure on the Kim Jong-un regime have been stepped up recently, following new missile tests and a large-scale military drill by the North.

Foreign Secretary Boris Johnson has said Britain stands “shoulder to shoulder” with Japan in efforts to put an end to North Korea’s nuclear and missile tests.

He said in Tokyo on Friday that “we all need to increase the pressure on Pyongyang through diplomacy and sanctions, and that must include China using its influence to bring North Korea back to the negotiation table”.

Mr Johnson is meeting Japanese Prime Minister Shinzo Abe, the foreign minister and the governor of Tokyo on day two of a three-day trip to Japan.

The Japanese foreign ministry says they are discussing increased diplomatic and security co-operation and the Olympics.

Mr Johnson called North Korea’s recent launch of its first intercontinental ballistic missile “a reckless provocation”.

Additional reporting by agencies