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 UN special rapporteur on human rights in the Democratic People's Republic of Korea (DPRK), said in a statement issued in Geneva.
Mr 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.
"His ordeal could have been prevented had he not been denied basic entitlements when he was arrested, such as access to consular officers and representation by an independent legal counsel of his choosing," added Mr Quintana, a lawyer and veteran UN rights expert.
North Korea is believed to operate political prison camps and foreign nationals have also been detained on political grounds, Ojea Quintana said. Two American university professors in Pyongyang were arrested this year for allegedly plotting anti-state acts.
A 2014 landmark report by a UN investigators catalogued massive human rights violations in North Korea which they said could amount to crimes agianst humanity.
Tens of thousands of people are detained across the isolated country in inhumane conditions and subjected to torture and forced labour, it said.