US President Donald Trump does not think North Korean leader Kim Jong-un is to blame for what happened to American prisoner Otto Warmbier.

Warmbier, an American student, was arrested in North Korea in 2016, accused of stealing a propaganda poster, and sentenced to 15 years of hard labour.

The North Korean government released him in June 2017 but when he returned to the US, doctors said he was in a coma. The 22-year-old died a few days later.

At a press conference on Thursday following his summit with Kim, Trump said: "Some really bad things happened to Otto — some really, really bad things. But he tells me that he didn't know about it and I will take him at his word."

The president added that Kim told him that he "felt very badly about it."

Trump said Kim had nothing to gain from the Warmbier situation: "I really don't think it was in his interest at all."

Otto's parents, Fred and Cindy Warmbier, were told he had been in a coma since shortly after being sentenced. The Warmbiers recalled some of the harrowing details of seeing Otto aboard a military aircraft in Cincinnati, where he was transported after his release.

"We got halfway up the steps, we heard this howling, involuntary, inhuman sound. We weren't really certain what it was," Fred Warmbier said.

"Otto was on the stretcher… and was jerking violently making these inhuman sounds," he added, noting that his son was blind and deaf.

In December, a US judge ruled that Warmbier's parents were entitled to more than €438 million in damages from North Korea.