White House national security adviser John Bolton said North Korea should provide a "full explanation" of what exactly transpired in the case of American student Otto Warmbier, who died after being released from North Korean captivity.

"The president's been very clear. He viewed what happened to Otto Warmbier as barbaric and unacceptable, and I think the best thing North Korea could do right now would be to come up with a full explanation of exactly what happened to him," Bolton said on "Face the Nation" Sunday.

Warmbier died in 2017 at the age of 22 after he was released in a coma following more than a year in captivity in North Korea. Doctors who treated Warmbier said he suffered severe injuries to all areas of his brain during his detention, causing "extensive loss of brain tissue in all regions of the brain." Brain scans indicated he suffered the injury shortly after his conviction in March 2016.

At a press conference in Vietnam after meeting with North Korean leader Kim Jong Un, President Trump suggested he did not believe Kim Jong Un was responsible for Warmbier's death, telling reporters he took Kim "at his word" that he didn't know about Warmbier's condition at the time.

"I don't believe that he would have allowed that to happen," Mr. Trump said. "Just wasn't to his advantage to allow that to happen. Those prisons are rough. They're rough places. And bad things happen. But I really don't believe he knew about it."

Mr. Trump later tried to walk back those comments, writing on Twitter: "Of course I hold North Korea responsible ... for Otto's mistreatment and death."

On Sunday, Bolton said the president first discussed the circumstances surrounding Warmbier's capture and death with Kim at the leaders' first summit in Singapore in 2018.

"I've heard the president talk about Otto Warmbier on any number of occasions in the Oval Office. And I know how strongly he feels about it. I have no doubt of that whatever," Bolton said.

In a statement following the summit, Warmbier's parents Fred and Cindy Warmbier called the North Korean regime "evil" and squarely placed blame on Kim for their son's death.

"We have been respectful during this summit process. Now we must speak out. Kim and his evil regime are responsible for the death of our son Otto. Kim and his evil regime are responsible for unimaginable cruelty and inhumanity. No excuses or lavish praise can change that," the statement read.