Trump and the ever-more-powerful Chinese president spoke by phone, the White House said, after the US leader stunned the world by accepting an invitation to meet Kim Jong Un before the end of May.
Trump and Xi committed to "maintain pressure and sanctions until North Korea takes tangible steps toward complete, verifiable, and irreversible denuclearization," according to the White House.
As aides scrambled to catch up with Trump's decision -- which was taken before consulting key aides -- the White House sent mixed messages about conditions for the talks.
"They've made promises to denuclearize, they've made promises to stop nuclear and missile testing," White House spokeswoman Sarah Huckabee Sanders said.
"We're not going to have this meeting take place until we see concrete actions that match the words and the rhetoric of North Korea," Sanders told reporters.
Officials behind the scenes said this did not constitute a change of heart.
A day after the bombshell announcement that the US and North Korean leaders would meet before the end of May, Vice President Mike Pence said the White House would keep "maximum pressure" on Pyongyang and claimed US efforts to isolate Kim had been vindicated.
There has been limited reaction from Kim's regime, but South Korean President Moon Jae-in said news of the summit -- announced by his national security advisor on a visit to Washington -- was "like a miracle."
For his part Xi urged the two leaders to begin talks as "soon as possible" and praised Trump's "positive aspiration".
China has long been North Korea's most important ally but has been on board with the sanctions agreed at the United Nations.
- 'Huge gamble' -
The UN Security Council has imposed tough economic sanctions aimed at choking off revenue to Pyongyang's military programs after Kim's regime carried out a sixth nuclear test and advanced missile launches.
China -- Pyongyang's sole ally -- and Russia argue that sanctions alone will not push North Korea to change course and have repeatedly called for stepping up diplomatic efforts to achieve a solution.
The summit announcement triggered a rise in global stock markets while world leaders voiced hope the meeting would deflate tensions that had been building dramatically in recent months.
Some observers questioned the US president's wisdom in granting Kim a long-standing wish for a meeting after only agreeing to temporarily halt nuclear tests.
Bill Richardson, former US ambassador to the United Nations, said it's a "huge gamble" but "worth taking."
"This is not 'The Apprentice' or a reality TV event," he added. "It's a negotiation with an unpredictable leader who has at least 20 nuclear weapons and who threatens the United States."
Republican Senator Cory Gardner told AFP America's approach to North Korea still needs not just a drop "but a whole bucket of reality."
"The challenge here is there is no greater diplomatic tool or lever than the President of the United States. In terms of the length of the diplomatic runway, if this doesn't succeed how much is left of that diplomatic runway is a very good question"
- 'Little Rocket Man' -
Trump has previously ridiculed Kim as "Little Rocket Man", slapping wideranging bilateral sanctions on the Pyongyang regime and also leading a drive for international sanctions through the UN.
Vice President Pence stressed that the White House has made "zero concessions" to get Kim to the table and emphasized that the sanctions would remain in place.
"North Korea's desire to meet to discuss denuclearization -- while suspending all ballistic missile and nuclear testing -- is evidence that President Trump's strategy to isolate the Kim regime is working," Pence said in a statement.
"Our policy remains the same: all sanctions remain in place and the maximum pressure campaign will continue until North Korea takes concrete, permanent, and verifiable steps to end their nuclear program."
German Chancellor Angela Merkel was among the world leaders to hail the announcement as a "glimmer of hope," saying North Korea's nuclear drive "has been a source of great concern for all of us."
The International Atomic Energy Agency, the UN's nuclear watchdog, voiced hope the summit would produce "concrete progress" and a resumption of long-suspended nuclear inspections.
Pyongyang's long race to develop a nuclear weapon capable has proved a problem for successive US administrations.
But the alarm bells have been ringing even louder since last July when Pyongyang conducted two intercontinental ballistic missile tests, declaring the entire United States now within range.
Trump threatened "fire and fury" if Pyongyang continued to threaten the United States only for North Korea to carry out its sixth nuclear test.
The United States and North Korea fought on opposite sides of a bloody war in the 1950s, and in the last two decades have been engaged in perhaps the world's most dangerous nuclear standoff, with 30,000 US military personnel stationed just over the border in the South.