"We are a long way from conclusion on North Korea, maybe things will work out, and maybe they won't - only time will tell," Trump said on Twitter on Sunday.

North Korea said on Saturday it was suspending nuclear and missile tests and scrapping its nuclear test site, and instead pursuing economic growth and peace.

World leaders welcomed the announcement, but some expressed doubts about Kim's intentions and South Korean President Moon Jae-in will be under intense international scrutiny when he meets him on Friday at the first North Korea-South Korea summit in more than a decade.

In a tweet minutes before he tempered enthusiasm about the North's statement, Trump interpreted it as a pledge to denuclearise: "Wow, we haven't given up anything & they have agreed to denuclearization (so great for World), site closure, & no more testing!"

However, Kim's announcement did not include a commitment to scrap existing nuclear weapons and missiles, and there are doubts he would ever give up the nuclear arsenal his country has been developing for decades.

Trump administration officials told the Wall Street Journal the US would not grant meaningful sanctions relief before Pyongyang substantially dismantles its nuclear programs.

"When the president says that he will not make the mistakes of the past, that means the US will not be making substantial concessions, such as lifting sanctions, until North Korea has substantially dismantled its nuclear programs," it quoted a senior Trump administration official as saying on Sunday.

Kim said North Korea no longer needed to test nuclear bombs or intercontinental ballistic missiles now that his country had the weapons, and he would gear all efforts toward economic development.

With past failures in mind, some expressed concern about the North's intentions.

"North Korea has a long history of raising the issue of denuclearisation and has committed to freeze its nuclear weapons programs in the past. We all remember how those pledges and commitments went down over past decades," said Nam Sung-wook, a professor of North Korean Studies at Korea University in Seoul.

In Washington, Bob Corker, the Republican chairman of the Senate Foreign Relations Committee, said the US government viewed Kim's pledge with scepticism.

"This is a great public relations effort by Kim Jong Un," Corker said on CNN. "I think everyone within the administration and Congress approaches this with scepticism and caution."

Kim is expected to meet Trump in late May or early June.

South Korea said the North's testing pledge signified "meaningful" progress that would create good conditions for successful summits with Seoul and Washington.

Moon, who welcomed Kim's announcement as a "major" step toward denuclearisation, is making Friday's summit his sole focus this week, an official said on Sunday.

Australian Associated Press