Washington's ban on US citizens travelling to North Korea came into force today, with the two countries at loggerheads over Pyongyang's weapons ambitions. The measure was imposed following the death of student Otto Warmbier, 22, in June, a few days after he was sent home in a mysterious coma following more than a year in prison in the North. <br/><br/>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 administration of US President Donald Trump over its banned missile and nuclear weapons programmes.<br/><br/>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.