WASHINGTON/SEOUL (REUTERS) - US President Donald Trump said the North Korean nuclear crisis is a long way from being resolved on Sunday (April 22), striking a cautious note a day after the North's pledge to end its nuclear tests raised hopes before planned summits with South Korea and the United States.

"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.

North Korea said on Saturday (April 21) 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, which came before the first North Korea-South Korea summit in more than a decade on Friday (April 20).

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 denuclearisation (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.

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.

Some have expressed doubts about the North's intentions and South Korean President Moon Jae-in will be under intense international scrutiny when he meets North Korean leader Kim Jong Un on Friday.

"North Korea has a long history of raising the issue of denuclearisation and has committed to freeze its nuclear weapons programmes 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.

"Although the North's announcement is quite dramatic, it's natural for the world to be extra sensitive to every word spoken by Kim." In Washington, Bob Corker, the Republican chairman of the Senate Foreign Relations Committee, said the U.S. government viewed Kim's pledge with skepticism.

"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 skepticism and caution."

Kim is expected to meet Trump in late May or early June, the first meeting between sitting leaders of the two countries. CIA Director Mike Pompeo, Trump's nominee to become the next secretary of state, secretly visited North Korea and met with Kim to discuss the summit three weeks ago.

Senator Tom Cotton, told CBS' "Face the Nation" he hoped the three Americans detained by North Korea would be released before any meeting. The Republican was unimpressed by the North's overture, saying it "is better than continued testing but it's not much better than that." U.S. officials say North Korea had in the past repeatedly reneged on denuclearisation agreements, the latest in 2012 when the North launched a long-range rocket after agreeing to a moratorium on missile testing.

PROGRESS OR EMPTY PROMISES?

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, a Blue House official said on Sunday.

For the past few weeks, South Korea has been renovating Peace House, on its side of Panmunjom, to prepare for the summit with Kim, who will be the first North Korean leader to set foot in the South since the 1950-1953 Korean War.

Moon now has a direct phone link with Kim on his office desk, instead of having to communicate through a hotline at the Joint Security Area in Panmunjom, which had been the main channel between the two sides over the Winter Olympics in February.

The two leaders are expected to talk over the newly installed phone for the first time this week, before the summit, South Korea said on Friday.

A senior US diplomat for East Asia, Susan Thornton, called North Korea's latest announcement "a very positive step" as she started a three-day visit to Seoul on Sunday.

"We are going to be doing a lot of close coordination with South Korea, allies and partners this week," the Yonhap news agency quoted her as saying.

Max Boot, a foreign policy adviser to Republican Mitt Romney's 2012 presidential campaign, warned that Kim was outmaneuvering the U.S. president.

"He's stringing Trump along, making vague promises that he has no intention of keeping. And Trump, the rube who thinks he's a sophisticate, shows every sign of falling for the bait," he wrote in a Washington Post opinion piece.

Corker said Washington needed to be clear-headed about what a meeting with Kim could accomplish.

"To think that somebody's going to go in and charm him out of that is not realistic," he told the ABC This Week programme.

"Is there some progress that can be made? I hope so, but that's a big hurdle."