An American college student serving a 15-year prison term in North Korea who was released and medically evacuated from the country on Tuesday has been in a coma for months, his parents said.

The announcement on Otto Warmbier's release came as former NBA player Dennis Rodman was paying a return visit to Pyongyang.

Mr Rodman is one of few people to have met both North Korean leader Kim Jong Un and President Donald Trump.

However, he said the issue of several Americans detained by North Korea is "not my purpose right now".

Secretary of State Rex Tillerson said the State Department had secured Mr Warmbier's release at the direction of the president.

He said Mr Warmbier, 22, of Cincinnati, Ohio, was en route to the US, where he will be reunited with his family.

Mr Tillerson made no mention of Mr Rodman's visit, and said the department would have no further comment on Mr Warmbier and his condition, citing privacy concerns.

Fred and Cindy Warmbier said in a statement to The Associated Press that their son is in a coma and flying home.

They said they have been told their son has been in a coma since March 2016 - when he was last seen in public, at his trial when he was sentenced to hard labour - 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, they said.

"We are so grateful that he will finally be with people who love him."

A North Korean foreign ministry official said Mr Warmbier was released and left the country on Tuesday morning.

It was not immediately clear if Mr Rodman's visit to North Korea was purely coincidental with Mr Warmbier's release.

Mr Rodman has travelled to the isolated nation four times previously.

It is, however, his first trip since Mr Trump, his former Celebrity Apprentice boss, became president.

He told reporters in Beijing, as he departed for Pyongyang, that he hopes his trip will "open a door" for Mr Trump.

The Trump administration sought to dampen speculation about Mr Rodman's role by sharing details of its diplomatic efforts to win consular access and freedom for Americans held in Pyongyang.

Joseph Yun, the US envoy on North Korea, met with North Korean foreign ministry representatives in Norway last month, a White House official said.

Such direct consultations between the two governments are rare as they do not have formal diplomatic relations.

At the meeting, North Korea agreed that Swedish diplomats could visit all four American detainees, which at that time included Mr Warmbier.

Mr Yun then met last week with the North Korean ambassador at the UN in New York, where he learned about Mr Warmbier's condition.

Mr Yun was then dispatched to North Korea and visited Mr Warmbier with two doctors on Monday, and demanded his release on humanitarian grounds.

Mr Tillerson said that the State Department is continuing "to have discussions" with North Korea about the release of the other three American citizens who are jailed there.

Mr 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 as he tearfully confessed that he had tried to steal a propaganda banner.

The US government condemned the sentence and accused North Korea of using such American detainees as political pawns.

The North Korean court held that Mr Warmbier had committed a crime "pursuant to the US government's hostile policy towards (the North), in a bid to impair the unity of its people after entering it as a tourist".

Mr Warmbier had said he tried to steal a propaganda banner as a trophy for an acquaintance who wanted to hang it in her church.

That would be grounds in North Korea for a subversion charge.

He identified the church as the Friendship United Methodist Church.

In a tearful statement before his trial, Mr Warmbier told a gathering of reporters in Pyongyang he was offered a used car worth 10,000 US dollars if he could get a propaganda banner and was also told that if he was detained and did not return, 200,000 dollars would be paid to his mother in the form of a charitable donation.

Mr Warmbier said he accepted the offer because his family was "suffering from very severe financial difficulties".

AP