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

Key points: Former US diplomat says Warmbier has been in a coma for a year

Former US diplomat says Warmbier has been in a coma for a year Warmbier had been sentenced to 15 years of hard labour

Warmbier had been sentenced to 15 years of hard labour US remains in discussions about three other Americans detained in North Korea

Warmbier, 22, a University of Virginia student from suburban Cincinnati, has been in a coma for over a year, 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.

"[He] urgently needs proper medical care in the United States," he 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," Mr Richardson, whose Centre for Global Engagement had directly sought Warmbier's release with the North Korean Government, said in a statement.A

t a US Senate hearing on Tuesday, Secretary of State Rex Tillerson declined to comment on Warmbier's condition.

The New York Times quoted a senior US official as saying Washington recently received intelligence reports that Warmbier had been repeatedly beaten in custody.

Sorry, this video has expired It was not the first occasion former basketball star Dennis Rodman visited the country. (Photo: Reuters/Kyodo)

The release came after Joseph Yun, the State Department's special envoy to North Korea, travelled to Pyongyang and demanded his freedom on "humanitarian grounds", capping a flurry of secret diplomatic contacts, a US official said.

Mr Yun reportedly arrived on Monday, visited Warmbier with two doctors, and demanded his release, the official said. The North Koreans agreed and he was flown out on Tuesday, the official said.

Warmbier's release also 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 were quick 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.

"This is him freelancing," the official said.

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

US Senator condemns North Korea's 'abhorrent behaviour'

Otto Warmbier was one of four Americans detained in North Korea. ( Reuters: Kyodo )

Since taking office in January, US President Donald Trump has faced growing tensions with North Korea, which has conducted a series of ballistic missile tests in defiance of US and international sanctions.

Warmbier was detained in January 2016 and sentenced to 15 years of hard labour in March last year for trying to steal a propaganda banner, 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," Mr Tillerson said.

Mr Richardson, a former Democratic congressman, UN ambassador, US energy secretary and ex-governor of New Mexico, welcomed Warmbier's release but said "we are deeply concerned regarding his health".

"Otto's detainment and sentence was unnecessary and appalling, and North Korea should be universally condemned for its abhorrent behaviour. Otto should have been released from the start," US Senator Rob Portman of Ohio said.

"Our son is coming home," Fred Warmbier told the Washington Post.

"At the moment, we're just treating this like he's been in an accident. We get to see our son Otto tonight."

Reuters