An American college student whose parents say has been in a coma while serving a 15-year prison term in North Korea was released and returned to the United States as the Trump administration revealed a rare exchange with the reclusive country.

An airplane carrying Otto Warmbier, who's from Ohio, arrived in Cincinnati shortly before 10:20 pm (local time). Two ambulances were parked near an airport hangar.

Warmbier's release came during a visit to North Korea by former NBA star Dennis Rodman, one of few people to have met both North Korean leader Kim Jong Un and President Donald Trump.

Department of State spokeswoman Heather Nauert told reporters Rodman had nothing to do with Warmbier's release. Rodman had told reporters before arriving in Pyongyang that the issue of Americans detained by North Korea is "not my purpose right now."

Securing Warmbier's release "was a big priority" for President Donald Trump, who worked "very hard and very closely" with Secretary of State Rex Tillerson, spokeswoman Sarah Huckabee Sanders said.

While North Korea's move to free Warmbier could potentially provide an opening for talks on security issues, the prospects still appear bleak. International negotiations on the dispute over North Korea's nuclear program have been in limbo for years, as the US cranks up economic sanctions and North Korea won't give up weapons it considers a guarantee against invasion.

The detention of Americans, often sentenced to draconian prison sentences for seemingly small offences in the totalitarian nation, has compounded tensions between Washington and Pyongyang. Three Americans remain in custody.

Warmbier, a 22-year-old University of Virginia undergraduate, was convicted and sentenced in a one-hour trial in North Korea's Supreme Court in March 2016. He was sentenced to 15 years in prison with hard labour for subversion after he tearfully confessed that he had tried to steal a propaganda banner.

Tillerson announced that the Department of State had secured Warmbier's release at the direction of the Republican president. He said Warmbier, of Wyoming, in suburban Cincinnati, was en route to the US.

Warmbier's parents, Fred and Cindy Warmbier, said he was in a coma and was flying home. They said they were told he has been in a coma since his trial, when he was last seen in public, and they had learned of this only one week ago.

"We want the world to know how we and our son have been brutalised and terrorised by the pariah regime" in North Korea, Warmbier's parents said. "We are so grateful that he will finally be with people who love him."

In Wyoming, resident Amy Mayer said news of his release had sent waves of shock and joy through the neighbourhood. A White House official said Trump had instructed Tillerson to take all appropriate measures to secure the release of Americans held in North Korea. The official referred to them as "hostages."