Trump: US paid no money over Warmbier

US President Donald Trump says the United States did not pay any money to North Korea as it sought the release of comatose student Otto Warmbier.

The Washington Post reported on Thursday Trump approved a $US2 million payment to North Korea to cover its care of Warmbier, who died shortly after he was returned to the US after 17 months in a North Korean prison.

The Post said an invoice was handed to State Department envoy Joseph Yun hours before Warmbier, 22, was flown out of Pyongyang in a coma on June 13, 2017. Warmbier died six days later.

The US envoy, who was sent to retrieve Warmbier, signed an agreement to pay the medical bill on instructions passed down from Trump, the Post reports.

"We do not comment on hostage negotiations, which is why they have been so successful during this administration," White House spokeswoman Sarah Sanders told Reuters before Trump's remarks.

Warmbier, a University of Virginia student visiting North Korea as a tourist, was imprisoned there from January 2016.

North Korea state media said he was sentenced to 15 years of hard labour for trying to steal an item bearing a propaganda slogan from his hotel.