Two of the men, agricultural expert Kim Hak-song and former professor Tony Kim were arrested in 2017, while Kim Dong-chul, a South Korea-born American businessman and pastor in his 60s, was sentenced to 10 years' hard labor in 2016.

The White House said all three men were able to walk unassisted onto a US Air Force plane that carried them and the secretary of state out of North Korea.

A second plane, with more robust medical equipment, awaited them at Yokota Air Force Base, just outside Tokyo and will bring them back to the United States.

"All indications are at this point that their health is as good as could be given that they've been held," Pompeo told reporters traveling with him.

The trio later released a statement of thanks conveyed via the State Department.

"We would like to express our deep appreciation to the United States government, President Trump, Secretary Pompeo, and the people of the United States for bringing us home," they said.

"We thank God, and all our families and friends who prayed for us and for our return. God Bless America, the greatest nation in the world."

The family of Tony Kim also voiced gratitude to "all of those who have worked toward and contributed to his return home" -- and specifically thanking Trump for "engaging directly with North Korea."

Game on

Trump described the men's release as "a gesture of good will" and said he would be on hand when Pompeo's three "guests" land shortly after 2:00 am (0600 GMT) Thursday.

Trump acolytes declared their release an unbridled political victory, evidence, Vice President Mike Pence said, that "strong leadership and our America First policies are paying dividends."

The development appears to pave the way for a much-anticipated summit between Trump and North Korean leader Kim, scheduled to take place within weeks.

"It was absolutely imperative that the Trump Administration secure the release of the three Americans well before any summit," said Jean Lee, an analyst at the Wilson Center.

In his first reported comments on the summit, North Korean leader Kim Jong-un said his sit down with Trump "would be a historic meeting" and an "excellent first step" according to state media.

Summit details to follow

Trump says a time, date and location have been decided for his historic summit with Kim, although US officials say there is still some fine-tuning to be done.

The most likely location appears to be neutral Singapore.

The president told reporters the administration would announce the details "within three days" but revealed it would not be at the Demilitarized Zone that separates North and South Korea.

"I think that Chairman Kim is trying to set good conditions for the summit. I think we are having good conversations, productive conversations," Pompeo said.

"We're planning on it will be a single day, but in the event that there is more to discuss, there'll be an opportunity for it to extend into the second day as well."

The meeting will discuss North Korea's nuclear and ballistic weapons program, which may soon give Pyongyang the capability of striking the continental United States.

Trump has demanded that North Korea give up its nukes, while Kim has offered few indications about what he is willing to concede or what he will demand in return.

North Korea has often insisted that the United States withdraw support for South Korea, where 30,000-plus US military personnel are stationed.

Since coming to office Trump has embarked on a campaign of "maximum pressure" on the North Korean regime.

A series of provocative North Korean missile tests have been met with US sanctions and a tougher Chinese stance on cross border trade.

Officials in Washington believe the punitive economic measures have prompted fuel shortages in North Korea and increased tension inside the regime.

North Korea official Kim Yong-chul, who met Pompeo in Pyongyang, insisted the country's openness to talks was "not the result of sanctions that have been imposed from outside," but a change in regime focus.

"We have perfected our nuclear capability. It is our policy to concentrate all efforts into economic progress in country," he said. (AFP)