Otto Warmbier, 22, has a severe brain injury and is in a state of "unresponsive wakefulness", his Ohio doctors said on Thursday.
His family said he had been in a coma since March 2016, shortly after he was sentenced to 15 years' hard labour in North Korea.
"While I welcome the news of Mr Warmbier's release, I am very concerned about his condition, and the authorities have to provide a clear explanation about what made him slip into a coma," Tomas Ojea Quintana, the U.N. special rapporteur on human rights in the Democratic People's Republic of Korea (DPRK), said in a statement issued in Geneva.
Ojea Quintana, speaking by telephone, later told Reuters: "There seems to be a disproportionate relation between the act or crime that Otto Warmbier is accused of and the penalty imposed on him from a human right points of view. There is a serious concern in this respect."
Warmbier, from a Cincinatti suburb, was arrested for trying to steal an item bearing a propaganda slogan, North Korean media reported. On Thursday, North Korea said that it had released him "on humanitarian grounds".
The University of Virginia student's father, Fred Warmbier, said his son had been "brutalised and terrorised" by the North Korean government.
Fred Warmbier said the family did not believe North Korea's story that his son had fallen into a coma after contracting botulism and being given a sleeping pill.
Ojea Quintana called on North Korea to "clarify the causes and circumstances" of Otto Warmbier's release.
Two other U.S. nationals, both university professors in Pyongyang, were arrested this year for allegedly plotting anti-state acts and remain in custody, he said.
"It is most important that wherever they are being held someone can meet them other than seeing them at a public trial. This is something I have been trying to do for foreign national prisoners," he told Reuters, urging they be granted access to consular officials. "This is where the problem lies."
Veteran former U.S. politician and diplomat Bill Richardson also offered on Friday to visit North Korea to try to secure their release.
A 2014 landmark report by a U.N. investigators catalogued massive human rights violations in North Korea which they said could amount to crimes against humanity.
Tens of thousands of people are detained across the isolated country in inhumane conditions and subjected to torture and forced labour, it said.
North Korea has "categorically and totally" rejected the U.N. report.
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