WASHINGTON (AFP) - North Korea has freed three American detainees, United States President Donald Trump announced on Wednesday (May 9), hailing a diplomatic victory ahead of a planned summit with Pyongyang leader Kim Jong Un.

Pyongyang granted the three men "amnesty", a US official said, and they are now on their way back to the US with secretary of state Mike Pompeo.

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

"I am pleased to inform you that Secretary of State Mike Pompeo is in the air and on his way back from North Korea with the 3 wonderful gentlemen that everyone is looking so forward to meeting. They seem to be in good health," Mr Trump tweeted.

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, waited for them at Yokota Air Force Base, just outside Tokyo.

"All indications are their health is as good as could be given they been through," said Mr Pompeo.

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

"Mostly, we thank God for Tony's safe return," the family said in a statement. "We ask that you continue to pray for the people of North Korea and for the release of all who are still being held."

Mr Trump meanwhile described the release as "a gesture of good will" and said he would be on hand when Mr Pompeo's three "guests" land at Andrews Air Force Base outside Washington at 2am (0600 GMT) on Thursday (May 10).

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

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

Mr Trump says a time, date and location have been decided for that historic summit, although US officials say there are still some details to be worked out.

Mr Pompeo told reporters the administration would "in the next handful of days be able to announce date and time".

Possible locations include Singapore and the Demilitarized Zone that separates North and South Korea.

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

Mr Trump has demanded that North Korea give up its nukes, while Mr 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 US withdraw support for South Korea, where 30,000-plus US military personnel are stationed.

Since coming to office, Mr 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 Mr 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.

WHO ARE THE THREE US CITIZENS FREED?

1. Kim Hak Song

Mr Kim Hak Song had been working for the Pyongyang University of Science and Technology (PUST) undertaking agricultural development work with the school's farm. He was arrested at Pyongyang railway station in May 2017 on suspicion of committing "hostile acts" against the government, as he was boarding a train headed for his home in Dandong, China.

Mr Kim, who is in his mid 50s, was born in Jilin, China, and educated at a university in California, CNN reported, citing a man who had studied with him. He said Mr Kim returned to China after about 10 years of living in the US.

PUST was founded by evangelical overseas Christians and opened in 2010. It is known to have a number of American faculty members and students are generally children from the North's elite.

2. Mr Tony Kim

Mr Tony Kim, or Kim Sang Duk, was arrested in April 2017 at the capital's main airport as he tried to leave the country after teaching for several weeks, also at PUST.

Mr Kim is a former professor at Yanbian University of Science and Technology in China, close to the Korean border. Its website lists his speciality as accounting.

South Korea's Yonhap news agency has reported Mr Kim is in his late 50s and said he had been involved in relief activities for children in rural parts of North Korea. It cited a source who described him as a "religiously devoted man".

In a Facebook post, Mr Kim's son said since his arrest his family has had no contact with him. His family said Mr Kim will soon become a grandfather.

3. Mr Kim Dong Chul

Mr Kim Dong Chul, a South Korea-born American businessman and pastor who is in his 60s, was sentenced to 10 years' hard labour in April 2016 after being charged with subversion and espionage.

He was arrested in October 2015 after he reportedly received a USB stick containing nuclear-linked data and other military information from a former North Korean soldier.

In an interview with CNN in January 2016, Mr Kim said he was a naturalised American who had lived in Fairfax, Virginia. He said he once ran a trading and hotel services company in Rason, a special economic zone near the North's border with China and Russia.

A month before his trial, Mr Kim had also appeared at a government-arranged news conference and apologised for attempting to steal military secrets in collusion with South Korea. The South Korean spy agency has denied involvement.