American college student Otto Warmbier was released and evacuated from North Korea on Tuesday after being held captive in the reclusive state for 17 months.

Warmbier's release came on the back of a rare diplomatic intervention in the nuclear-armed state, after the US State Department's special envoy on North Korea, Joseph Yun, travelled to Pyongyang to demand his freedom on "humanitarian grounds."

Read more: Ex-NBA star Dennis Rodman heads to North Korea 'to open the door'

US Secretary of State Rex Tillerson said the State Department had "secured" Warmbier's release, adding that his agency was pushing for three more Americans imprisoned in North Korea to be freed. It was not clear whether the US had been forced to make any concessions.

State Department spokeswoman Heather Nauert also said it was "just too soon to say" whether Warmbier's release could lead to more dialogue between Washington and Pyongyang.

Warmbier, a 22-year-old undergraduate student at the University of Virginia, was sentenced to 15 years in prison by North Korea's Supreme Court in March 2016 after tearfully confessing to trying to steal a political banner from a hotel.

Warmbier in a coma

Following the evacuation, Bill Richardson, a veteran former diplomat who has played a role in past talks with North Korea, revealed after speaking to Warmbier's parents that the student had been in a deep coma for more than a year and "urgently needs proper medical care in the United States."

According to US media, Fred and Cindy Warmbier, Otto's parents, were told by North Korean officials that their son had suffered from botulism following his trial and fallen into a coma after taking a sleeping pill.

Read more: Can North Korea's elites oust Kim Jong Un?

In a statement, the family said: "We want the world to know how we and our son have been brutalized and terrorized by the pariah regime. We are so grateful that he will finally be with people who love him."

Richardson said that US officials had only found out about Warmbier's condition a week earlier and that "in no uncertain terms, North Korea must explain the cause of his coma."

White House dismisses Rodman role in release

Warmbier's release coincided with a visit to the reclusive state by former basketball player Dennis Rodman, likely to be the only mutual friend shared by both US President Donald Trump and North Korea leader Kim Jong Un.

Rodman – a former contestant on Trump's Celebrity Apprentice television show – has travelled to the reclusive state four times since 2013 as part of a quixotic mission to broker a detente deal between the US and North Korea.

The State Department denied that there was a connection between Warmbier's release and the former NBA star's controversial visit. It also said that the Trump administration did not authorize Rodman's visit and warned Americans against going to North Korea.

When quizzed by reporters in Beijing while en route to the North, Rodman said that the issue of Americans detained by Pyongyang was "not my purpose right now." However, he added that Trump would be happy with the trip as he was "trying to accomplish something we both need," prompting speculation he could potentially be serving the administration as an unofficial envoy.

dm/rt (AP, AFP, Reuters)