Washington - The Trump administration will ban American citizens from travelling to North Korea, US officials said on Friday, following the death of university student Otto Warmbier who died in June after falling into a coma in a North Korean prison.Secretary of State Rex Tillerson had decided to impose "geographical travel restriction" for North Korea, the officials said, which would make it illegal to use US passports to enter the country. They said the restriction would be published in the Federal Register next week and will take effect 30 days after that.The officials were not authorised to publicly discuss the decision before it is announced and spoke on condition of anonymity. Two tour operators that organise group trips to North Korea said they had already been informed of the decision.It wasn't clear how many Americans the move will effect, as figures about how many Americans go to North Korea are difficult for even the US government to obtain. The US strongly warns Americans against travelling to North Korea, but has not until now prohibited it despite other sanctions targeting the country. Americans who venture there typically travel from China, where several tour groups market trips to adventure-seekers.Barring Americans from stepping foot in North Korea marks the latest US step to isolate the furtive, nuclear-armed nation, and protect US citizens who may be allured by the prospect of travelling there. Nearly all Americans who have gone to North Korea have left without incident. But some have been seized and given draconian sentences for seemingly minor offences.The travel ban comes as the Trump administration searches for more effective ways to ramp up pressure on North Korea over its nuclear weapons programme. WarmbierUnder US law, the secretary of state has the authority to designate passports as restricted for travel to countries with which the United States is at war, when armed hostilities are in progress, or when there is imminent danger to the public health or physical security of United States travellers. Americans who violate the restriction could face a fine and up to 10 years in prison for a first offence.Warmbier, who died after being medically evacuated in a coma from North Korea last month, suffered a severe neurological injury from an unknown cause while in custody. Relatives said they were told the 22-year-old University of Virginia student had been in a coma since shortly after he was sentenced to 15 years of hard labour in North Korea in March 2016. He had been accused of stealing a propaganda poster while on a tour of the country.The United States, South Korea and others often accuse North Korea of using foreign detainees to wrest diplomatic concessions. At least three other Americans remain in custody in the North.