Otto Warmbier, the US student released by North Korea in a coma after more than a year in detention, has died, his family said.

The 22-year-old, who had suffered severe brain damage, was medically evacuated to the United States on June 13.

He died Monday at 2.20 pm, local time, (2.20am Tuesday AEST) surrounded by family at a hospital in his home town of Cincinnati, Ohio.

"It is our sad duty to report that our son, Otto Warmbier, has completed his journey home," his family said in a statement.

"The awful torturous mistreatment our son received at the hands of the North Koreans ensured that no other outcome was possible."

Pyongyang said that Mr Warmbier fell into a coma soon after he was sentenced in March last year for stealing a political poster from a North Korean hotel.

Speaking moments after news of Otto Warmbier's death was announced publicly, US President Donald Trump hit out at Pyongyang.

"It's a brutal regime," the president said during a White House event.

"Bad things happened but at least we got him home to his parents."

In a separate written statement, Mr Trump offered his "deepest condolences" to Mr Warmbier's family "on his untimely passing."

"There is nothing more tragic for a parent than to lose a child in the prime of life. Our thoughts and prayers are with Otto's family and friends, and all who loved him."

"Otto's fate deepens my Administration's determination to prevent such tragedies from befalling innocent people at the hands of regimes that do not respect the rule of law or basic human decency."

"The United States once again condemns the brutality of the North Korean regime as we mourn its latest victim."

Mr Warmbier's release came amid mounting tensions with Washington following a series of missile tests by Pyongyang, focusing attention on an arms buildup that Pentagon chief Jim Mattis has dubbed "a clear and present danger to all".

Mr Warmbier's family said they were only told about the coma two weeks ago, and called for justice.

"Even if you believe their explanation of botulism and a sleeping pill causing the coma - and we don't," his father, Fred Warmbier, told a news conference last week in the family's home, "there is no excuse for any civilised nation to have kept his condition secret and denied him top-notch medical care for so long".

"I call on them to release the other Americans being held," he urged, referring to three US citizens still in North Korea.

Before his death, Mr Warmbier's medical team said he was completely unresponsive.

"He has spontaneous eye opening and blinking. However, he shows no signs of understanding language, responding to verbal commands or awareness of his surroundings," neurologist Daniel Kanter said.

The medical team said Mr Warmbier's severe brain injury was most likely - given his young age - to have been caused by cardiopulmonary arrest cutting the blood supply to the brain.