EXPLAINER-Top issues for U.S. in planned Trump summit with North Korea's Kim

(Adds White House on conditions for lifting sanctions, paragraph 18)

By David Brunnstrom and John Walcott

WASHINGTON, April 22 (Reuters) - U.S. administration officials are preparing for an unprecedented summit between President Donald Trump and North Korean leader Kim Jong Un, but basic details, including where and when it will happen and negotiating tactics, are still being worked out.

The summit would be the first-ever meeting between a serving U.S. president and a North Korean leader, and will follow one between South Korean President Moon Jae-in and Kim on Friday. Trump has said his meeting with Kim could take place in late May or June but has warned it could be called off if he did not think it could deliver the desired results.

Prospects were boosted on Saturday when Kim announced an end to nuclear and missiles tests, saying North Korea was scrapping its nuclear test site and pursuing economic growth and peace. U.S. and South Korean officials say Kim has said he is willing to discuss denuclearization, but he said nothing on Saturday about giving up his existing nuclear weapons.

Trump and other world leaders welcomed Kim's announcement of an end to testing, but some expressed doubts about North Korea's intentions. The U.S. president said on Sunday the crisis over its pursuit of nuclear missiles capable of hitting the United States was still a long way from being resolved.

"Maybe things will work out, and maybe they won’t - only time will tell," Trump said on Twitter.

Trump's administration has said it wants North Korea's "complete, verifiable, and irreversible denuclearization" but has offered few details of the strategy it will employ in talks.

It has vowed not to repeat mistakes of the past. Decades of unsuccessful engagement with Pyongyang, however, help define the contours of a negotiating process that could last years.

DENUCLEARIZATION

Getting Pyongyang to abandon its nuclear missile programs has been the aim of all international negotiations with North Korea since the early 1990s, yet last year it tested what is widely believed to have been an H-bomb and intercontinental ballistic missiles.

Trump's CIA director and secretary of state nominee, Mike Pompeo, said this month he was optimistic a Trump-Kim summit could set a course for a diplomatic outcome with North Korea, but no one was under any illusion that a comprehensive deal could be reached at the meeting.

Pompeo, who spoke just after becoming the first serving U.S. official ever to meet Kim, suggested U.S. interests would be put first, saying the aim would be “an agreement...such that the North Korean leadership will step away from its efforts to hold America at risk with nuclear weapons.”