WASHINGTON (REUTERS, AFP) - US President Donald Trump on Thursday (June 7) held out the prospect of inviting North Korean leader Kim Jong Un to the White House if he deemed next week's summit in Singapore a success while also signalling he was willing to walk away if he thought talks did not go well.

"Certainly if it goes well. I think it would be well received," Trump said in answer to a question on inviting Kim to Washington. "I think he would look at it very favourably so I think that could happen."

But the US president also reiterated he would quit the talks if he felt he must, and would ramp up US sanctions pressure on North Korea if the talks did not go well. "I am totally prepared to walk away," he said.

At a White House news conference with Japanese Prime Minister Shinzo Abe, Trump repeated what he said last Friday that it was possible he and Kim could sign an agreement to end the 1950-53 Korean War, which was concluded only with a truce, not a peace treaty.

"We could sign an agreement, as you know that would be a first step... We're looking at it, we're talking about it with a lot of other people," Trump told reporters on Thursday. "That's probably the easy part. The hard part remains after that."

Trump added that he hoped someday US relations with Kim's secretive Pyongyang government could be normalised.

He said a letter he received last week from Kim was "warm".

"It was a warm letter and nice and I appreciated it very much," Trump said.

The US president said the June 12 summit was "all ready to go".

The main issue for the summit in Singapore is the US demand for North Korea to abandon a nuclear weapons programme that now threatens the United States. North Korea has rejected giving up its arsenal unilaterally.

North Korea defends its nuclear and missile programmes as a deterrent against what it sees as US aggression.

The United States stations 28,500 troops in South Korea, a legacy of the Korean War.

US Secretary of State Mike Pompeo on Thursday re-emphasised Washington's stance going into the talks. Pompeo said Trump will reject anything short of "complete, verifiable and irreversible denuclearisation of the Korean Peninsula.

"President Trump is hopeful. But he's also going into the summit with his eyes wide open," Pompeo said at a White House briefing after the Trump-Abe news conference.

Pompeo, however, added that the US would work to guarantee North Korea's security should it denuclearise. Trump "is prepared to ensure a DPRK free of its weapons of mass destruction is also a secure North Korea," Pompeo said.

Pompeo plans to stay in the region following the summit to meet with officials from Japan and South Korea and to travel to China, an important North Korean ally, to discuss the next steps involving Pyongyang.

Pompeo, who has twice met with Kim in Pyongyang, said he believes the North Korean leader is prepared to take "bold" steps to alter the course of his nation.

"He has indicated to me personally that he is prepared to denuclearise," Pompeo said. "That he understands that the current model doesn't work, that he's prepared to denuclearise and that two, he understands that we can't do it the way we've done it before. That is has to be big and bold and we have to agree to making major changes."

Asked whether the two sides had narrowed their differences on how they defined denuclearisation, Pompeo said they had, but he declined to give details.

US officials said Kim had indicated he was willing to make a pledge to denuclearise, but had never made it clear if this would mean simply halting development of the weapons programme where it stands or also disposing of its existing nuclear arsenal.

The officials, who spoke on condition of anonymity, said they expect that the Singapore summit will yield nothing more than a vague "end of war declaration" and leave the main issues on defining denuclearisation for further negotiations.

North Korea has argued for a "phased" approach to denuclearisation of the entire Korean peninsula, which in the past has also meant removal of the U.S. nuclear umbrella protecting Washington's allies South Korea and Japan.

At the meeting with Abe, Trump said he promised to raise with Kim the subject of Japanese citizens abducted by North Korea, after Abe focused on the issue during an earlier conversation.

"He talked about it long and hard and passionately and I will follow his wishes and we will be discussing that with North Korea absolutely, absolutely," Trump said.