US student held prisoner by North Korea dies days after release

American university student Otto Warmbier, who was released in a coma last week after nearly 18 months in detention in North Korea, died on Monday, prompting President Donald Trump to slam the "brutal regime" in Pyongyang.

Warmbier, 22, was arrested in North Korea while visiting as a tourist, and sentenced in March last year to 15 years hard labour for stealing a political poster from a North Korean hotel, a punishment the United States decried as far out of proportion to his alleged crime.

North Korea released Warmbier last week and said he was being freed "on humanitarian grounds."

He had been described by doctors caring for him last week as having extensive brain damage that left him in a state of "unresponsive wakefulness."

He died six days later surrounded by relatives in his hometown of Cincinnati, Ohio.

"The awful torturous mistreatment our son received at the hands of the North Koreans ensured that no other outcome was possible beyond the sad one we experienced today," the family said in a statement announcing Warmbier's death at 7:20pm.

His family has said that Warmbier lapsed into a coma in March 2016, shortly after he was sentenced to 15 years of hard labour in North Korea.

Physicians at the University of Cincinnati Medical Center, where he died, said last Thursday that Warmbier showed no sign of understanding language or awareness of his surroundings, and had made no "purposeful movements or behaviours," though he was breathing on his own.

There was no immediate word from Warmbier's family on the cause of his death.

US President Donald Trump lashed out at Pyongyang following news of his death.

"It's a brutal regime," he said during a White House event. "Bad things happened but at least we got him home to his parents."

In a separate written statement, Trump said, "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.”

The circumstances of Warmbier's detention in North Korea and what medical treatment he may have received there remained a mystery, but relatives have said his condition suggested that he had been physically abused by his captors.