WASHINGTON/SEOUL - Singapore is the mostlikely venue for a summit between US President Donald Trump and Kim Jong-un, top leader of the Democratic People’s Republic of Korea, a US official said, after Pyongyang cleared a major obstacle to the meeting with the release of three American prisoners.

The three prisoners, freed after US Secretary of State Mike Pompeo met Kim in the DPRK capital, were flying back to Washington where Trump said he planned to meet them early on Thursday.

The release of the men appeared to signal an effort by Kim to improve the tone for the summit and followed his recent pledge to suspend missile tests and shut a nuclear bomb test site.

A US official, speaking on the condition of anonymity, said Singapore has emerged as the likeliest location for the planned summit after Trump ruled out holding it at the heavily fortified demilitarised zone between North and South Korea.

Trump, who previously said Singapore was under consideration, said agreement had been reached on a date and venue and details would be announced within three days.

The release of the prisoners gave Trump a chance to tout a diplomatic achievement just a day after his decision to pull out of the Iran nuclear deal drew heavy criticism from European allies and others.

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

"I appreciate Kim Jong-un doing this and allowing them to go," Trump told reporters at the White House.

The three men are Korean-American missionary Kim Dong-chul, detained in 2015; Kim Sang-duk, also known as Tony Kim, who spent a month teaching at the foreign-funded Pyongyang University of Science and Technology (PUST) before he was arrested in 2017; and Kim Hak-song, who also taught at PUST and was detained last year.

State media of the DPRK said they were arrested either forsubversion or "hostile acts" against the government.

Reuters