US President Donald Trump said on Thursday he will meet Democratic People's Republic of Korea leader Kim Jong-un on June 12 in Singapore.

"The highly anticipated meeting between Kim Jong Un and myself will take place in Singapore on June 12th," the president wrote on Twitter. "We will both try to make it a very special moment for World Peace!"

Trump posted the message hours after he greeted at the Joint Base Andrews outside Washington the three US citizens who were released by the DPRK.

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

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.