US President Donald Trump envisioned "a very special moment for world peace" as he announced his meeting with Kim Jong-un, leader of the Democratic People's Republic of Korea, which he said is set for 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 announced his face-to-face with Kim on Twitter Thursday morning, hours after he greeted at the Joint Base Andrews outside Washington the three US citizens who were released by the DPRK.

"I think we have a very good chance of doing something very meaningful," Trump said at the airport. "My proudest achievement will be — this is part of it — when we denuclearize that entire peninsula."

Singapore's Ministry of Foreign Affairs confirmed late Thursday the time and location of the planned Trump-Kim summit.

"We hope this meeting will advance prospects for peace in the Korean Peninsula," the ministry said in a statement.

The Southeast Asian city-state was chosen for the site — out of a total of five initial candidates — mostly because of its location, security advantages and track record of hosting international summits, observers have said.

In a speech on the floor of the US Senate, Democratic Leader Chuck Schumer warned Trump against going too far too fast in Singapore.

"I worry that this president, in his eagerness to strike a deal and get the acclaim and a photo op, will strike a quick one and a bad one, not a strong one, not a lasting one," Reuters quoted Schumer as saying.

The White House said in a statement on Thursday that freeing the three detainees is "a step in the right direction", as negotiations continue ahead of the upcoming Trump-Kim meeting.

The DPRK's "complete, verifiable and irreversible denuclearization" is Trump's top priority in talks with Kim, the White House said.

US Senator Ted Cruz also tweeted after his meeting with South Korean Ambassador to the US Cho Yoon-je on Thursday, "We must remain committed to securing the complete, verifiable, and irreversible denuclearization of North Korea."

The diplomatic progress follows the shockwaves Trump sent around the world on Tuesday when he announced that the US was withdrawing from a landmark international nuclear deal with Iran.

The move raised questions over whether the DPRK might now be less inclined to negotiate its own nuclear deal with Washington, Reuters reported on Thursday.

On Wednesday, US Defense Secretary Jim Mattis said the US troops stationed in South Korea would not be part of initial negotiations with the DPRK.

"That's not something that would be on the table in the initial negotiation," Mattis told a congressional hearing.

Eliminating security threats has been one of the sought-after goals of the DPRK.

In his meeting with President Xi Jinping in the northeastern Chinese coastal city of Dalian earlier this week, Kim said it has been the DPRK's consistent and clear stance to achieve denuclearization of the Korean Peninsula.

"As long as relevant parties abolish their hostile policies and remove security threats against the DPRK, there is no need for the DPRK to be a nuclear state, and denuclearization can be realized," he said.

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