The three United States detainees released are Korean-American missionary Kim Dong-chul, Kim Sang-duk - also known as Tony Kim - and Kim Hak-song. "But we expect that to be announced here in the next few days".
"They were happy to be with us on this plane, to be sure", Pompeo told reporters during the flight.
Following Kim Jong Un's historic meeting last month with the South Korean leader, at which they agreed on complete denuclearisation, Mr Trump warned that the U.S. would "not be played" when it came to talks with North Korea.
Pompeo was briefly in the country this week making plans for the leaders' upcoming summit.
Pompeo, in his visit, discussed the agenda for a potential Trump-Kim summit in a meeting with Kim Yong Chol, vice chairman of the central committee of North Korea's ruling party.
Mr. Warmbier, an undergraduate at the University of Virginia, had been convicted in March 2016 of trying to steal a propaganda poster while on a trip to North Korea and sentenced to 15 years of hard labor.
According to Chinese state television, Xi said he "hopes the United States and North Korea can build mutual trust" and "resolve each sides' concerns through meeting and consultations". "They seem to be in good health", the President wrote in a post on Twitter. Current reports explained that the men were transferred out of a labour camp to your hotel outside their North Korean capital of Pyongyang, but the State Department couldn't validate the stories' validity.
Then he announced that he'd sent Secretary of State Mike Pompeo to Pyongyang to lay the groundwork for Trump's historic summit with the dictatorial Kim Jong Un, recently described by Trump as "very honorable".
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Bolton himself said the USA was looking for a "real deal" with Pyongyang that would include "the elimination of both the front and the back end of the nuclear fuel cycle; no uranium enrichment; no plutonium reprocessing". Kim Dong Chul, president of a company involved in global trade and hotel services, was sentenced to 10 years on espionage charges.
Kim Yong Chul noted improved relations between North and South Korea and pushed back against the idea that USA pressure led to the likely summit. He worked at Pyongyang University of Science and Technology, and it was not clear whether his arrest was connected with that of his colleague Tony Kim two weeks earlier.
During his incarceration, Warmbier sustained a neurological injury and fell into a coma before being released into US custody last summer. -North Korea tension, already running high at the time over Pyongyang's stepped-up missile tests.
The three men, of Korean descent, were suspected to be the last American hostages left in North Korea, according to The New York Times.
He said the regime realized that releasing the prisoners was the right move, and it would buy them substantial global goodwill.
Kim met South Korean President Moon Jae-in at the end of April at a building in Panmunjom known as the Peace House.
But the US president's decision Tuesday to pull Washington out of the 2015 accord with Teheran has rattled experts, who say it undermines the image of Washington as a reliable treaty partner.
But strong worldwide sanctions in response to a major nuclear test in September, and several missile tests which suggest the North is capable of targeting the majority of the US, have pushed Pyongyang to engage diplomatically, King added.