The North Korean flag flutters next to concertina wire at the North Korean embassy in Kuala Lumpur, Malaysia, March 9, 2017. REUTERS/Edgar Su/Files

WASHINGTON: Journalists and humanitarian workers may apply for exceptions to a new ban on US citizens' travel to North Korea, the State Department said in a public notice on Wednesday.

The US government last month said it would bar Americans from travelling to North Korea due to the risk of "long-term detention" in the country.

American student Otto Warmbier, sentenced last year to 15 years' hard labour in North Korea, returned to the United States in a coma on June 13 after being released on humanitarian grounds and died June 19.

The circumstances surrounding his death are not clear, including why he fell into a coma.

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 State Department issued a notice in the Federal Register on Wednesday declaring US passports invalid for travel to, in or through North Korea. The restriction takes effect in 30 days and applies for one year unless extended or revoked by the secretary of state.

Professional reporters or journalists, representatives of the International Committee of the Red Cross or the American Red Cross traveling on official missions, those traveling to North Korea for "compelling humanitarian considerations", and those whose requests are "in the national interest" may ask for a special validation of their passports in order to travel to the country, the State Department said.

North Korea is currently holding two Korean-American academics and a missionary, a Canadian pastor, and three South Korean nationals who were doing missionary work.

Japan says North Korea has also detained at least several dozens of its nationals.