Secretary of State Rex Tillerson confirmed the release, and said Washington remains in talks with the isolated regime "regarding three other US citizens reported detained."
Officials could not immediately confirm US media reports that Warmbier has fallen into a coma during his incarceration.
"We have no comment on Mr Warmbier's condition, out of respect for him and his family," Tillerson told US senators at the start of a hearing into his budget.
Warmbier was detained at the airport as he was leaving the country with a tour group in January 2016.
The United States has accused the North of using Warmbier as a political pawn and condemned the sentence as far out of proportion to his alleged crime.
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