SEOUL, South Korea (AFP) — Washington’s ban on US citizens travelling to North Korea came into force on Friday, with the two countries at loggerheads over Pyongyang’s weapons ambitions.

The measure was imposed following the death of student Otto Warmbier in June, a few days after the 22-year-old was sent home in a mysterious coma following more than a year in prison in the North.

He had been convicted of offences against the state for trying to steal a propaganda poster from a Pyongyang hotel and sentenced to 15 years’ hard labour, with President Donald Trump blaming Pyongyang’s “brutal regime” for his plight.

On its website the State Department said it took the decision due to “the serious and mounting risk of arrest and long-term detention of US citizens”.

Three Americans accused of various crimes against the state are behind bars in the North, which is engaged in a tense standoff with the Trump administration over its banned missile and nuclear weapons programmes.

Earlier this week Pyongyang launched a missile over Japan, in a major escalation, and it has threatened to fire rockets towards the US Pacific territory of Guam.

In July it carried out its first two successful tests of an intercontinental-range missile, apparently bringing much of the US mainland into range.

Exemptions to the travel ban are available for journalists, Red Cross representatives, those travelling for humanitarian purposes, or journeys the State Department deems to be in the national interest of the United States.

But NGOs working in the North privately express concerns about how the process will function and the potential impact on their work.

A few remaining US citizens in the country left on Thursday, reports said.