Mr. Trump, who spent much of the day talking to other Asian leaders about the high stakes of his encounter with Mr. Kim, told Singapore’s prime minister, Lee Hsien Loong, that he was bullish about the outcome.
“I just think it’s going to work out very nicely,” he said.
While Mr. Trump had lunch with Mr. Lee under a glittering chandelier in the prime minister’s ceremonial offices, a team of American diplomats was meeting at a nearby hotel to try to lock down the language of a joint communiqué to be issued by Mr. Trump and Mr. Kim at the end of their meeting.
The statement is likely to have three sections — dealing with denuclearization, security guarantees for the North and steps to be taken by both sides — according to a person briefed on the talks. But it was not clear that the Americans would succeed in extracting a more detailed commitment to disarming than North Korea has already offered.
f anything, the White House moved closer to Pyongyang’s language, saying it sought complete, verifiable and irreversible denuclearization “on the Korean Peninsula” — a clause that North Korea interprets as potentially requiring the United States to scale back troop deployments there or to shrink its nuclear umbrella over two East Asian allies, South Korea and Japan.
Secretary of State Mike Pompeo insisted the administration’s policy had not changed. But he confirmed that the United States would offer security assurances that were different from what Washington had offered North Korea in negotiations under Presidents George W. Bush or Bill Clinton. He declined to outline them.
“We’re prepared to take actions that will provide them sufficient certainty that they can be comfortable that denuclearization isn’t something that ends badly for them — indeed, just the opposite, that it leads to a better, brighter future for the North Korean people,” he said.
“The concept for these discussions is radically different than ever before,” Mr. Pompeo added.
That is largely because of the personal involvement of Mr. Trump and Mr. Kim, both mercurial figures with a gift for self-promotion and a hunger to be actors on the world stage. But these talks are also different because of the progress that North Korea has made in the past year on its arsenal, especially the development of a missile that can strike the mainland United States.
On Monday evening, the White House announced that Mr. Trump planned to leave Singapore on Tuesday evening, after a single day of talks with Mr. Kim. It was not clear whether the move was tactical — to put pressure on the North Korean leader — or reflected a growing recognition on the part of the White House that the two men may have little to talk about.
Singapore’s government has turned this futuristic city into a giant stage set for Mr. Trump and Mr. Kim. On Tuesday, they will meet in a hotel on the island of Sentosa, where tourists and locals go to visit the Universal Studios theme park or the crescent-shaped beach.
About 2,500 journalists from around the world are on hand to chronicle a summit meeting that some officials predict will amount to the world’s most extravagant meet-and-greet exercise. Even if it is successful, Mr. Pompeo predicted, it will only inaugurate a lengthy, complicated and risky process.
For Mr. Trump, Monday was a brief intermission between the tumult of an acrimonious Group of 7 meeting in Canada over the weekend and the looming spectacle of his encounter with Mr. Kim.
Mr. Trump stayed largely out of sight in the heavily guarded Shangri-La Hotel, where he has mostly been closeted with aides since landing in Singapore on Sunday evening. Less than a mile away, as if in a rival armed camp, Mr. Kim hunkered down at his own fortified hotel, the St. Regis.
Aside from lunch with Mr. Lee, and a greeting to American diplomats based here, White House officials said, Mr. Trump’s day was occupied by last-minute cramming for the Kim meeting, as well as phone calls with Prime Minister Shinzo Abe of Japan and President Moon Jae-in of South Korea.
But Mr. Trump refused to let go of his rancorous clash with European allies over trade. On Monday morning, from his hotel, he unleashed a fusillade of angry posts on Twitter about what he said were the predatory trade practices of Canada and several European countries.
“Sorry, we cannot let our friends, or enemies, take advantage of us on Trade anymore,” the president said in a tweet. “We must put the American worker first!”
Mr. Trump’s harsh words about the nation’s closest allies stood in stark contrast with his expression of sunny feelings toward Mr. Kim, a brutal dictator who, only a few months ago, threatened the United States with a nuclear attack and traded bitterly personal insults with Mr. Trump.
“Great to be in Singapore, excitement in the air!” tweeted Mr. Trump, who had yet to set foot outside his hotel since arriving.
While Mr. Trump consulted with his closest aides — including Mr. Pompeo; the chief of staff, John F. Kelly; and the national security adviser, John R. Bolton — the diplomats negotiating at the Ritz-Carlton Hotel were nearing the end of their work on a joint statement, Mr. Pompeo said.
What remained unclear is whether that statement would include timelines for North Korea to give up its weapons, a detailed accounting of the scale of the North’s nuclear program or a reference to sanctions, which Pyongyang wants lifted. Among the other steps that could be taken is an American commitment to open a diplomatic outpost in Pyongyang.
The administration recruited Sung Y. Kim, a seasoned North Korea negotiator currently serving as American ambassador to the Philippines, to lead that effort. Mr. Kim, the ambassador, and a small group of diplomats held a series of talks last week with the North Koreans in the town of Panmunjom, the so-called truce village in the Demilitarized Zone between North and South Korea.
People briefed on the meetings said American negotiators found it difficult to make significant headway with the North Koreans, in part because the White House did not back them up in taking a hard line.
In his public statements, Mr. Trump has shown gradually greater flexibility toward North Korea, saying he viewed its disarmament as a “process,” rather than something to be done all at once, and disavowing the phrase “maximum pressure,” after making it the centerpiece of his policy.
Mr. Pompeo took issue with a report in The New York Times that Mr. Trump would be handicapped in the negotiations because of a lack of scientists on his negotiating team. He said the government could draw on the expertise of dozens of people with doctorates in nuclear, chemical and biological weapons.
“Any suggestion that the United States somehow lacks the technical expertise across government, or lacks it on the ground here in Singapore, is mistaken,” Mr. Pompeo said.
Some foreign-policy experts said the breakdown at the Group of 7 meeting would play to North Korea’s advantage, since Mr. Trump can ill afford two failed summit meetings, back to back. The president has consistently predicted success, even as his definition of that has grown foggier.
The unraveling of the gathering in Canada increases the North Korean leader’s incentive to “up his asks and limit his compromises and for Trump to do the opposite,” Richard Haass, the president of the Council on Foreign Relations, said on Twitter, adding, “Hardly the ideal context.”
Still, other analysts said Mr. Kim was as determined as Mr. Trump to make this meeting a success. That, as much as Mr. Trump’s need for a win after Canada, may guarantee a positive outcome.
“The underlying driver is a transformational process of Kim Jong-un leading North Korea into a new place in the region and the world,” said John Delury, a professor of Chinese studies at Yonsei University in Seoul. “Kim and Trump both seem to want the same thing: a dramatic reversal in the U.S.-North Korea relationship, which can be attributed to their vision.”
South Korea’s president, Mr. Moon, underlined the historic nature of the meeting. Denuclearizing North Korea could take years, he said, warning that Mr. Kim and Mr. Trump cannot end decades of hostility through a single meeting.
Mr. Moon, who worked tirelessly to help mediate this first-ever summit meeting, urged a “bold give-and-take” to make it successful. But he said that regardless of whatever agreement was produced, it would be just the beginning of what could be a long, bumpy process of ridding North Korea of a nuclear arsenal it has spent decades building.
“Even after the two heads of state open the gate,” Mr. Moon said, “it will take a long process to achieve a complete solution. We don’t know how long it will take: one year, two years or more.”