The 22-year-old contracted botulism last year and is in "bad shape", a source close to the family told CNN.
Warmbier was a University of Virginia student when he made a decision to go to North Korea with a tour group.
Bill Richardson, a veteran former diplomat and politician who has played a role in past negotiations with North Korea, said after speaking to Mr Warmbier's parents: "Otto has been in a coma for over a year now and urgently needs proper medical care in the United States".
"Sadly, he is in a coma and we have been told he has been in that condition since March 2016".
Secretary of State Rex Tillerson first announcedWarmbier's release and noted that the State Department is still working to free three other American citizens being held in North Korea.
The White House says a USA envoy met with North Korean foreign ministry representatives in Norway last month as part of efforts to win freedom for Americans held by Pyongyang.
Before his trial, Warmbier had said he tried to steal a propaganda banner as a trophy for an acquaintance who wanted to hang it in her church.
Emmett Saulnier, Warmbier's roommate at the University of Virginia, told VOA that Warmbier's parents called him early Tuesday, before news broke of Warmbier's release, to let him know his friend would be returning.
Warmbier's release Tuesday followed an extraordinary period of clandestine negotiations between US and North Korean officials in Oslo, Norway, and NY.
Whenever Rodman visits North Korea, he's usually greeted with a red carpet and lots of publicity. It wasn't immediately clear whether the release was connected to Rodman's visit.
Rodman begins sightseeing on NKorea trip he hopes opens door
A 2009 visit by former President Bill Clinton, a Democrat, secured the freedom of American journalists Euna Lee and Laura Ling. Rodman, 56, told CNN he was hoping to do "something that's pretty positive" as he boarded a plane to North Korea from Beijing.
An abrupt announcement came to American officials last week with news that Warmbier was in a coma, according to the Post.
Nauert also warned Americans against traveling to North Korea, a caution that former US government officials echoed.
Korean-American Kim Dong-Chul was arrested in 2015 and sentenced to 10 years' hard labour on charges of subversion and espionage.
At last check, Warmbier was on his way back to the U.S.to be reunited with his family.
The announcement comes as former National Basketball Association player Dennis Rodman is paying a return visit to North Korea.
After the Swedish diplomats met with the detained Americans, the North Koreans urgently asked to have a meeting with the U.S.in NY.
Americans are regarded as enemies in North Korea because the two countries never signed a peace treaty to formally end the 1950-53 Korean War. "No one", Fred Warmbier said. Three more Americans are still detained.
The trip comes at a time of heightened tensions between the USA and North Korea, which has conducted missile tests and detained four American citizens who are still in custody.
Secretary of State Rex Tillerson confirmed the release of the student, who was arrested in 2016 for stealing a propaganda sign, and said Washington remained in talks with the isolated regime "regarding three other USA citizens reported detained".
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