US President Donald Trump on Tuesday accused North Korea of brutally abusing an American student who had been held captive in North Korea, saying the young man had been "tortured beyond belief."

Otto Warmbier, 22, died in June a few days after he was sent home in a mysterious coma after more than a year in prison in North Korea.

Trump had previously blamed Pyongyang's "brutal regime" for Warmbier's plight but it was the first time the president publicly accused North Korea of torture in the case.

US officials have said Trump was personally taken aback by Warmbier's death and his comment Tuesday ratchets up the pressure on Kim Jong-Un's regime.

Warmbier 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.

"Otto was tortured beyond belief by North Korea," Trump said on Twitter.

His post followed the airing of an interview with Warmbier's parents.

"They kidnapped Otto, they tortured him, they intentionally injured him. They are not victims, they are terrorists," Fred Warmbier said Tuesday on "Fox and Friends."

The father had previously criticized former president Barack Obama's administration for how little it did for their son, saying officials had counselled the family against speaking out to avoid antagonizing the North Korean regime.

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 programs.