Trump told a joint news conference with Japanese Prime Minister Shinzo Abe on Wednesday that his campaign of "maximum pressure" on North Korea would continue until Pyongyang gave up its nuclear weapons.

He also said Washington was negotiating for the release of three Americans held by North Korea and there was "a good chance of doing it." He did not answer a reporter's question as to whether that would be a condition for going ahead with the summit.

"I hope to have a very successful meeting (with Kim)," Trump said in Palm Beach, Florida.

"If I think that it's a meeting that is not going to be fruitful, we're not going to go," he added. If the meeting when I'm there is not fruitful, I will respectfully leave the meeting."

Trump said earlier that Pompeo, one of his most trusted advisers and his pick to be the next US secretary of state, formed a "good relationship" with Kim when he became the first US official known to have met the North Korean leader.

US officials said Pompeo met Kim when he visited Pyongyang over the Easter weekend, which ran from March 31 to April 2, to lay the groundwork for the planned summit, in which Trump hopes to persuade North Korea to abandon development of nuclear missiles capable of hitting the United States.

"Mike Pompeo met with Kim Jong Un in North Korea," Trump tweeted earlier. "Meeting went very smoothly and a good relationship was formed. Details of Summit are being worked out now. Denuclearization will be a great thing for World, but also for North Korea!"

A senior administration official said Pompeo brought up the case of the three American prisoners with Kim in North Korea and that the United States was hopeful for their release.

Pompeo's visit provided the strongest sign yet of Trump's willingness to become the first serving US president to meet a North Korean leader.

Trump said on Tuesday he believed there was a lot of goodwill in the diplomatic push, which he has said could take place in late May or early June.

Australian Associated Press