President Donald Trump on Friday capped a week of whipsaw talks by reinstating a summit with North Korean leader Kim Jong Un just days after he had abruptly canceled it, but he also sought to lower expectations over the potential for a quick denuclearization deal.

Trump made the announcement in impromptu remarks outside the South Portico after meeting for more than 90 minutes with a top Kim aide in the Oval Office. Kim Yong Chol, the vice-chairman of North Korea's Central Committee, delivered a personal letter from the young dictator, a gesture viewed as an effort to ease tensions after Trump abruptly called things off last week amid escalating threats from Pyongyang.

But even as the president hailed the restart of his high-stakes diplomatic endeavor, he acknowledged that a full breakthrough on long-stymied U.S. efforts to eliminate the North's nuclear weapons program would be unlikely at the summit, set for June 12 in Singapore.

"I never said it goes in one meeting," Trump told reporters, after walking Kim Yong Chol to a black SUV outside the South Portico and taking pictures with him and Secretary of State Mike Pompeo. "I think it's going to be a process. But the relationships are building, and that's a very positive thing."

Trump characterized the summit - the first between a sitting U.S. president and a North Korean leader - as "a beginning" and a "getting to know you meeting-plus" in his effort to apply his unorthodox brand of personal diplomacy to a challenge that has vexed his predecessors.

"You're talking about years of hostility; years of problems; years of, really, hatred between so many different nations," Trump said. "But I think you're going to have a very positive result in the end. Not from one meeting."

The president's remarks suggested that his administration is coming to terms with the widely held view among former U.S. officials that Kim Jong Un has no intention of quickly relinquishing an arsenal his family has spent decades assembling.

The near-collapse of the summit, after a hostile response from Pyongyang to suggestions from Trump aides that the United States would demand a rapid denuclearization process, offered new evidence that any path to a deal is likely to be marked by fits and starts and threatened by potential land mines.

Past U.S. administrations have accused North Korea of violating agreements with additional nuclear and ballistic missile tests. Asked Friday if he was confident that the North Korean regime was committed to denuclearization, the president said: "I think they want to do that. I know they want to do that."

But Trump also suggested additional summit meetings with Kim could be necessary.

"I told them, 'I think that you're going to have, probably, others,' " Trump said. "'Hey, wouldn't it be wonderful if we walked out and everything was settled all of a sudden from sitting down for a couple of hours?' No, I don't see that happening. But I see over a period of time."

Experts said Trump's shifting rhetoric was necessary to keep the summit on track by reducing the gap in expectations between Washington and Pyongyang, which has signaled it would only negotiate over a slower, step-by-step process to curb its weapons programs in exchange for reciprocal benefits from the United States and other countries.

After Trump called off the summit in a letter to Kim last week, negotiating teams from the two sides have met in the Korean demilitarized zone and in Singapore to try to forge agreement over the summit's agenda and logistics.

Pompeo met with Kim Yong Chul for two hours in New York on Thursday, a prelude to the White House meeting Friday.

"We've seen communications from both sides over the last few weeks that reduce the gap," said Joseph Yun, who served as the State Department's special representative for North Korea policy until stepping down earlier this year. "Now Trump is talking about two or three summits; it's entirely possible they're not going to get done in one. Similarly, Secretary Pompeo is talking about process and progress."

The lengthy meeting with Trump marked the first time since 2000 - when President Bill Clinton met a top military liaison to Kim Jong Il, the current leader's father - that a North Korean official visited the White House. Eighteen years ago, Jo presented Clinton with a letter from the North Korean leader inviting him to a summit in Pyongyang, an offer Clinton ultimately turned down.

While Jo wore a military uniform, Kim Yong Chol - the former spy chief who is leading the North Korea side in pre-summit talks - was dressed in a dark business suit when he arrived at the South Portico shortly after 1 p.m. He was greeted by White House Chief of Staff John Kelly and Andrew Kim, a CIA official in charge of the agency's Korea Mission Center, who escorted him into the Oval Office.

Kim's visit represented an extraordinary turn of events. He was personally sanctioned by the United States over his role in the North's nuclear weapons program and is thought to have masterminded an attack that sank a South Korean naval vessel in 2010, killing 46 sailors. He needed a special waiver from the State Department to travel to New York and to Washington.

Although it's unusual for a president to meet in the Oval Office with foreign officials who are not heads of state, White House aides said the gesture was appropriate given that Kim Jong Un has met twice with Pompeo in Pyongyang over the past two months.

In speaking to reporters, Trump said he and Kim Yong Chol discussed the economic sanctions on the North and a potential agreement to formally end the Korean War. An armistice signed in 1953 has left the Korean Peninsula in a state of tension for more than six decades.

"I look forward to the day I can take sanctions off of North Korea," Trump said. "We talked about ending the war. This war has been going on - got to be the longest war, almost 70 years, right? A possibility of something like that."

In Seoul, a spokesman for South Korean President Moon Jae-in said with Friday's meeting at the White House, the road to a summit "appears to have expanded and strengthened."

"We will excitedly, but calmly, look forward to their historic meeting in Singapore," the spokesman added.

Experts have acknowledged that confidence-building measures are an important part of the negotiations, but they have warned against Trump moving too quickly to reward Kim without demonstrable concessions from Pyongyang.

Kim Jong Un "has more to show us that he is serious," Yun said. "What makes us believe North Korea is serious about changing direction? That's where we need to press them to prove that they're serious: What are the immediate steps? What is the timeline? What is the deadline?"