Otto Warmbier, a US university student held captive in North Korea for 17 months, has been released, but a former US official says he is in a coma and in urgent need of medical care.

"Otto has been in a coma for over a year now and urgently needs proper medical care in the United States," said Bill Richardson, a veteran former diplomat and politician who has played a role in past negotiations with North Korea, said after speaking to Warmbier's parents on Tuesday.

Warmbier, 22, a University of Virginia student from suburban Cincinnati, was on his way back to the United States, Secretary of State Rex Tillerson said.

The family said they were told by North Korean officials, through contacts with American envoys, that Warmbier fell ill from botulism sometime after his March 2016 trial and lapsed into a coma after taking a sleeping pill, the Washington Post reported.

"In no uncertain terms North Korea must explain the causes of his coma," said Richardson, whose Center for Global Engagement directly sought Warmbier's release with the North Korean government.

Tillerson, at a US Senate hearing on Tuesday, declined to comment on Warmbier's condition.

A person who answered the phone at Warmbier's family's Ohio residence said: "No comment, thank you" and hung up.

Warmbier's release came as former US basketball star Dennis Rodman arrived in North Korea on Tuesday, returning to the increasingly isolated nuclear-armed country where he has previously met leader Kim Jong Un.

US officials appeared to brush aside any speculation of a connection between Rodman's controversial visit and Warmbier's release.

A senior administration official said the Trump administration did not authorise Rodman's trip.

The State Department is continuing to discuss the situation of three other detained Americans with North Korea, Tillerson said.

Warmbier was detained in January 2016 and sentenced to 15 years of hard labour in March last year for trying to steal an item with a propaganda slogan, according to North Korean media.

"At the direction of the president, the Department of State has secured the release of Otto Warmbier from North Korea," Tillerson said.