US President Donald Trump has announced he will meet North Korean leader Kim Jong-un in Singapore on June 12.

"We will both try to make it a very special moment for World Peace!" Mr Trump said on Twitter.

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Mr Trump had previously said he favoured holding the meeting at the Demilitarised Zone (DMZ), the heavily fortified border separating the two Koreas.

But aides favoured the island nation, arguing the DMZ would look too much like Mr Trump going to Mr Kim's turf.

The two leaders are expected to discuss North Korea's nuclear weapons development and testing program, which has deepened long-seated tensions between Washington and Pyongyang.

Mr Trump's announcement came just hours after three Americans, who had been held prisoner in North Korea, arrived at a US military base outside Washington, having been released by Mr Kim.

Mr Trump said on their arrival that he believed Mr Kim wanted to bring North Korea "into the real world" and had high hopes for their planned meeting, which would be the first between a serving US president and a North Korean leader.

"I think we have a very good chance of doing something very meaningful," Mr Trump said.

"My proudest achievement will be — this is part of it — when we denuclearise that entire peninsula."

Mr Kim has suspended nuclear and missile tests and put his nuclear program up for negotiation, but questions remain about how serious his offer is and what disarmament steps he would be willing to take.

US Vice President Mike Pence said on NBC News on Thursday: "In this moment the regime in North Korea has been dealing, as far as we can see, in good faith."

Worry President is mostly interested in 'photo op'

New US Secretary of State Mike Pompeo has visited Pyongyang twice in recent weeks — once as head of the CIA — but there has been no sign that he cleared up the central question of whether North Korea will be willing to bargain away nuclear weapons that its rulers have long seen as crucial to their survival.

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

The Republican President, Senator Schumer said, should insist upon strong, verifiable commitments from North Korea on disarmament.

"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," Senator Schumer said.

The island-country of Singapore is friendly turf for Donald Trump. ( Flickr: Rajesh_India )

During Mr Trump's presidency, Mr Kim has overseen weapons tests that rattled the United States, South Korea and Japan as the North Korean leader attempted to showcase his military's progress on medium- and long-range missiles and atomic weapons.

Mr Trump has credited a US "maximum pressure" campaign for drawing North Korea to the negotiating table, and vowed to keep economic sanctions in place until Pyongyang takes concrete steps to denuclearise.

But former spy chief Kim Yong-chul, director of North Korea's United Front Department, said in a toast to Mr Pompeo over lunch in Pyongyang this week that the North had "perfected our nuclear capability".

"It is our policy to concentrate all efforts into economic progress … this is not the result of sanctions that have been imposed from outside," he said.

Mr Kim recently promised to suspend missile tests and shut a nuclear bomb test site.

North Korea is still technically at war with the United States and its ally South Korea because the 1950-53 Korean War ended in a truce, not a treaty.

Sorry, this video has expired Last month Kim Jong-un became the first North Korean leader to walk across the border between the North and South. (Image: AP)

Smiling and holding hands, Mr Kim and South Korean President Moon Jae-in held a rare round of talks at the heavily fortified demilitarised zone between the countries at the end of April, pledging to pursue peace after decades of conflict.

South Korea said on Thursday it welcomed the Singapore meeting and said it hoped "the denuclearisation of the Korean peninsula as well as permanent peace on the peninsula will successfully come about through this summit."

The choice of Singapore will put the summit on friendly turf for Mr Trump, as the island nation is a strong US ally and the US Navy frequently visits its port.

US officials had looked at several sites other than Singapore for the historic meeting but each was seen as problematic.

Reuters/AP