WASHINGTON/SEOUL - US President Donald Trump said on Sunday the North Korean nuclear crisis was a long way from being resolved, 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 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 denuclearize: "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.The Trump administration said on Sunday that Pyongyang would have to substantially dismantle its nuclear programs before U.S. concessions."We are looking for substantial dismantlement of North Korea's nuclear programs first, and until denuclearization is achieved the global maximum pressure campaign will continue," a White House National Security Council spokesman told Reuters.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."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 US 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.White House legislative affairs director Marc Short said on NBC's "Meet the Press" the U.S. goal remains "full denuclearization" in which Pyongyang no longer has nuclear weapons.