President Donald Trump will meet with South Korean President Moon Jae-in at the White House on Tuesday amid signs that his close partnership with Seoul in brokering a historic nuclear deal with North Korea's Kim Jong Un is faltering.

Moon's top aides presented Trump in March with the invitation from Kim to hold the unprecedented summit, an offer Trump accepted on the spot. But Moon's visit to Washington coincides with renewed tensions on the Korean Peninsula that have thrust doubt on the fate of the meeting, scheduled to take place in Singapore next month.

Trump advisers have expressed alarm at Pyongyang's hostile rhetoric and actions over the past week, questioning whether Kim is committed to pledges to seriously discuss denuclearization. The president, who spoke to Moon late Saturday, is expected to further press the South Korean leader on his views of Kim's willingness to change course on his nuclear program, a White House official said.

"This time last week, Moon was coming here with the intention of trying to heavily script what Trump would do in his meeting with Kim," said Victor Cha, who served as senior Asia director at the National Security Council in the George W. Bush administration. "Now, he's coming here just to try to save the summit. The mission has really changed."

For Moon, who has staked his presidency on the peace push, the White House visit, scheduled weeks ago, represents a crucial opportunity to soothe Trump's concerns and, perhaps, readjust his expectations about the potential outcome.

Trump has consistently raised public expectations over the summit, suggesting he would be able to secure a historic breakthrough with Kim where previous administrations have failed over 27 years of off-and-on negotiations and several past deals that quickly collapsed.

Yet the North Koreans, after taking steps to build confidence, including Kim's announcement that his regime would destroy a nuclear testing site, have reverted to threats to cancel the summit over objections to U.S.-South Korea joint military exercises and hard-line statements from Trump aides.

"Moon has to come in as assuager-in-chief and massage the situation between Trump and Kim," said Jung Pak, a former CIA official who now serves as an Asia analyst at the Brookings Institution. "North Korea's comments really have thrown some obstacles in the way of what seemed to be going pretty smoothly."

With just three weeks until the June 12 summit with Kim, major questions remain unresolved, including the agenda for the talks and logistics over the site for the meeting. White House aides declined to answer questions about what Trump planned to discuss with Moon.

Moon is scheduled to spend less than two hours at the White House, and he and Trump are not planning a joint news conference or public statement, according to aides.

The two leaders intend to "continue their close coordination," said Robert Palladino, a National Security Council spokesman.

Moon has played a major role in Trump's sharp turn from a war of words with Kim to the precipice of a possible summit. Alarmed at the escalating threats between the United States and North Korea last year, Moon intervened, using the Winter Olympics in South Korea in February to open new talks with Pyongyang that led to Kim's invitation to meet with Trump.

Yet Trump's decision to accept the offer without vetting it - he sent South Korean officials out to announce the news to reporters assembled in the West Wing driveway - has led to an accelerated timeline that some analysts have called a mistake.

With limited time to prepare, the Trump administration has struggled to work out key details with their Pyongyang counterparts. Neither side has explicitly defined their views of what denuclearization of the Korean Peninsula would look like, and Trump has at times contradicted his aides.

Last week, after Pyongyang lambasted national security adviser John Bolton for suggesting that the North would be expected to unilaterally relinquish its nuclear program, Trump undercut Bolton during impromptu remarks at the White House.

The president declared that Kim would be offered a deal that would make him "very, very happy."

The question, analysts said, is how much responsibility Moon bears for feeding Trump's unrealistic expectations - or whether Trump has failed to be properly skeptical amid talk from his supporters that he could win a Nobel Peace Prize if he strikes a deal in Singapore.

"I don't think Moon manipulated Trump. I think that Trump is guilty of some self-delusion here," said Frank Jannuzi, president of the Mansfield Foundation who made three trips to Pyongyang from 2001 to 2008 while serving as a Democratic aide on the Senate Foreign Relations Committee.

"Trump allowed himself to believe that denuclearization was on Trump's terms," Jannuzi said. The goal for Moon, he added, is to act as a "cheerleader" and convince Trump that "he can do a deal for peace and denuclearization and that it can be historic, but that it must be realistic. And in order for it to be realistic, Trump is going to have to appreciate that denuclearization will take time and that that's OK."

Moon is not the only other foreign leader with a major stake in the Trump-Kim summit. White House officials have expressed frustration over China's influence on Kim, suggesting that Pyongyang shifted back to a more belligerent tone after Kim's second visit to Beijing to meet with President Xi Jinping.

In a tweet Monday morning, Trump warned China to remain "strong and tight" in enacting economic sanctions imposed by the United Nations on North Korea, arguing that the border between those two countries "has become much more porous."

"I want this to happen, and North Korea to be VERY successful, but only after signing!" Trump wrote.

Bonnie Glaser, a China analyst at the Center for Strategic and International Studies, said Beijing would prefer that the U.S.-North Korea talks move forward lest Trump and Kim fall back into mutual threats of a military confrontation. But she said Xi is intent on ensuring that a potential deal does not shift Pyongyang away from Beijing's influence.

"China's biggest nightmare," Glaser said, "is North Korea having a closer relationship with the United States and South Korea than it does with China."