The momentum is building, and the timetable is set for a historic summit between the United States and the Democratic People's Republic of Korea, as Pyongyang released three US citizens on Wednesday, whom US President Donald Trump said he will greet early Thursday morning.

The three detainees, Kim Dong Chul, Kim Hak Song and Tony Kim, were released as US Secretary of State Mike Pompeo visited the DPRK on Wednesday to finalize plans for the summit. It was Pompeo's second trip around a month apart, during both of which he met with DPRK leader Kim Jong-un.

Trump said he appreciated Kim "doing this and allowing them to go" and he planned to greet the three at the Joint Base Andrews outside Washington early Thursday.

White House spokeswoman Sarah Sanders said in a statement that Trump viewed the release of the American detainees "as a positive gesture of goodwill".

The US president continued to parcel out information about his upcoming meeting with Kim, planned for this month or early next month. On Wednesday morning, he said the venue and timing for the summit had been hammered out.

"We've picked a time. We've picked a place for the meeting, or 'summit', as you like to call it," he said, again without specifying. But he ruled out the Korean Demilitarized Zone, a likely site that days before he said was "appealing".

He added that the timing of the summit will be announced "within three days".

Trump again thanked President Xi Jinping, saying that "China has been very helpful" on the DPRK.

During his visit to Pyongyang, Pompeo discussed the agenda for a potential Trump-Kim summit in his meeting with Kim Yong-chol, vice-chairman of the Central Committee of the ruling Workers' Party of Korea, The Associated Press reported on Wednesday.

The vice-chairman noted improved relations between North and South Korea and pushed back against the idea that US pressure led to the likely summit, according to the AP report.

"This is not a result of sanctions that have been imposed from outside," the AP cited Kim Yong-chul as saying. The report said that that contradicted Trump, who has said repeatedly that his pressure tactics brought the DPRK to the negotiating table.

Victor Cha, senior adviser and Korea chair at the Center for Strategic & International Studies in Washington, said the release of the three US detainees is a "welcome development and a useful confidence-building measure" leading up to the summit.

Pompeo's latest effort to finalize plans for the Trump-Kim summit, in which denuclearization is expected to be a core topic, coincided with Trump's announcement on Tuesday that he was withdrawing the US from a landmark international nuclear deal with Iran and re-imposing sanctions on the Islamic republic.

Antony Blinken, former deputy secretary of state under Barack Obama, said the White House move "makes getting to yes with North Korea that much more challenging".

"Why would Kim ... believe any commitments President Trump makes when he arbitrarily tears up an agreement with which the other party is complying?" Blinken said on Twitter.