An American college student who was released from a North Korean prison is finally home but in a coma and undergoing treatment at an Ohio hospital where he was taken shortly after arriving on U.S. soil.

An airplane carrying Otto Warmbier, who is from Ohio, landed in Cincinnati late Tuesday night. The 22-year-old was then taken by ambulance to the University of Cincinnati Medical Center.

Warmbier was serving a 15-year prison term with hard labour in North Korea for alleged anti-state acts.

In Warmbier's hometown of Wyoming, just outside of Cincinnati, residents helped tie blue and white ribbons, Wyoming High School's colours, to trees and said news of his release had sent waves of shock and joy through the community.

'A sense of relief'

"Everybody feels a sense of relief that he is coming back to the United States," resident Amy Mayer said before he arrived. "I think we're very excited yet very prayerful about what is happening because we've heard he is in a coma. So I think that people are trying to be supportive of the family and let the community-family know that we are very with them."

A hospital spokeswoman said Warmbier's family was expected to hold a news conference Thursday morning at Wyoming High School.

Warmbier had been sentenced to a 15-year prison term for trying to steal a banner. (Kyodo/Reuters)

Securing Warmbier's release "was a big priority" for U.S. President Donald Trump, who worked "very hard and very closely" with Secretary of State Rex Tillerson, spokesperson 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 U.S. cranks up economic sanctions and North Korea won't give up weapons it considers a guarantee against invasion.

Draconian prison sentences

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 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.

Warmbier's parents, Fred and Cindy Warmbier, 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 brutalized and terrorized 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."

Political pawns

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."

The U.S. government accuses North Korea of using such detainees as political pawns. North Korea accuses Washington and South Korea of sending spies to overthrow its government.

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 Trump, who was Rodman's boss on Celebrity Apprentice.

Department of State spokesperson Heather Nauert told reporters Rodman had nothing to do with Warmbier's release. Rodman, who has travelled to the isolated nation four times since 2013, had told reporters before arriving in Pyongyang that he hopes his trip will "open a door" for Trump.

Discussions about other prisoners

North Korea poses one of the greatest national security challenges for Trump as it tries to develop a nuclear-tipped missile that could strike America. He is looking to increase economic and diplomatic pressure on North Korea, with help from China, but has said he's open to meeting Kim.

In the past, North Korea has held out until senior U.S. officials or statesmen came to personally bail out detainees. A 2009 visit by former president Bill Clinton, a Democrat, secured the freedom of American journalists Euna Lee and Laura Ling.

Tillerson said the Department of State was continuing "to have discussions" with North Korea about the release of other three American citizens imprisoned there.