President Donald Trump insisted on Friday that the United States paid North Korea nothing for the release of Otto Warmbier, a young American student who fell into a coma after allegedly being tortured in the totalitarian country.

On Thursday, The Washington Post quoted unidentified sources as saying that a US official was made to sign a pledge to pay $2 million in medical costs before being allowed to fly Warmbier back home from Pyongyang in 2017.

The envoy signed the pledge on instructions from Trump, the Post reported.

But in a tweet Friday, Trump said: "No money was paid to North Korea for Otto Warmbier, not two Million Dollars, not anything else."

He then wrote, without saying where the quote came from: "President Donald J. Trump is the greatest hostage negotiator that I know of in the history of the United States. 20 hostages, many in impossible circumstances, have been released in last two years. No money was paid." Cheif (sic) Hostage Negotiator, USA!"