(CNN) -- The historic diplomatic encounter planned for three weeks from now between President Donald Trump and North Korean despot Kim Jong Un appeared in doubt on Tuesday as Trump questioned whether preparations could be completed in time.
"There's a very substantial chance that it won't work out," Trump said in the Oval Office, where he was sitting for critical talks with his South Korean counterpart. "That doesn't mean that it won't work out over a period of time, but it may not work out for June 12."
It was the clearest indication to date that the audacious summit Trump agreed to in March may be at risk. Last week, North Korea adopted a harsh new tone and threatened to withdraw from the meeting, which is due to occur in Singapore.
Hours after Trump spoke, his Secretary of State Mike Pompeo sought to cast a more optimistic view of the talks, appearing for the first time in the State Department briefing room to insist a June 12 meet was still the goal.
"We're working to make sure that there is a common understanding about the contents of what will be discussed. But I'm optimistic," Pompeo said. "It could be something that comes right to the end and doesn't happen. As the President said, we'll see. And that is the place that we find ourselves."
Trump also maintained that preparations were "moving along" for the talks with Kim; indeed, advance teams are surveying hotel ballrooms in Singapore as possible venues. But the President suggested there may not be enough time for the two sides to agree on mutually agreeable parameters.
"We'll see what happens," Trump said. "If it doesn't happen, maybe it'll happen later. Maybe it'll happen at a different time. But we are talking."
In his meetings Tuesday with South Korean President Moon Jae-in, Trump hoped to gain clarity on North Korea's nuclear intentions. Moon, meanwhile, was hoping to shore up confidence for the Kim meeting, which he helped to broker.
Some US officials believe Moon oversold Pyongyang's promises when his government relayed Kim's invitation to Trump for talks in March. At the time his envoy said North Korea was "committed to denuclearization," but recent statements from the North have cast doubts on Kim's willingness to negotiate away his nuclear weapons.
That, in turn, has led to skepticism the summit between Trump and Kim will proceed. White House aides have grown pessimistic in recent days that the talks will occur, and Trump underscored the uncertainty on Tuesday.
He again accused China of meddling in his diplomatic overtures with North Korea, saying Beijing was to blame for the new harsh tone from Pyongyang after Kim met in early May with Chinese President Xi Jinping.
"I think things changed after that meeting and I can't say I'm happy about it," Trump said.
But he insisted he believed Kim was earnest in his nuclear vows, and suggested Pyongyang had much to gain from striking a deal.
"We will guarantee his safety," he said of Kim. "He will be safe, he will be happy, his country will be rich, his country will be hardworking and very prosperous."
Moon, who has urged a diplomatic path in the belief it could forestall war, arrived in Washington in a bid to bolster confidence that the Singapore meeting will be a success. He met with Kim himself last month to great fanfare along the Korean Demilitarized Zone, a historic encounter that Trump hopes to replicate in his own talks.
Those plans were complicated last week when North Korea issued a series of harsh statements condemning joint US-South Korea military exercises and threatening to pull out of the Trump summit if the US continues to call for nuclear abandonment.
US officials were prepared to press Moon on the recent shift in tone, hoping to determine whether it is a signal of changing intentions or whether the North is simply trying to test Trump's willingness to negotiate ahead of the summit.
Ahead of the talks, Moon's representatives projected a positive message.
"We believe there is a 99.9% chance the North Korea-US summit will be held as scheduled," Chung Eui-yong, Moon's national security adviser, told reporters on the flight from Seoul to Washington. "But we're just preparing for many different possibilities."
US officials declined to offer their own prospects.
"I'm not a betting man. I wouldn't care to predict whether it would happen, only to predict that we'll be ready in the event that it does," Pompeo said.
Trump and Moon met in the Oval Office at noon before joining a larger working lunch with aides. There was no joint news conference, and Moon spent only two hours at the White House.
He did meet with some of Trump's aides earlier in the day, however, as questions about the administration's approach to the summit continue to mount.
National security adviser John Bolton, who has been outspoken in his hawkish views toward North Korea, drew Pyongyang's ire when he suggested Trump use a so-called "Libya model" to rid the country of its nuclear weapons. The US brokered a deal with Libyan leader Moammar Gadhafi in 2003 to abandon his nuclear weapons, but he was overthrown and killed in 2011.
The Libya suggestion raised eyebrows in Seoul, where Bolton's comments were deemed unhelpful at best and deeply damaging to the potential for diplomacy at worst. Trump later clarified that he wasn't pursuing the Libya model in North Korea, but speculated things could end poorly for Kim if he doesn't agree to a deal.
Pompeo has meanwhile adopted a more diplomatic approach, saying an agreement with Kim to abandon nuclear weapons could lead to economic assistance. Pompeo has met Kim twice in North Korea but didn't emerge with any specific commitments toward dismantling the nuclear program.
He said Tuesday he was open to returning to meet Kim if it was required to keep the summit with Trump on track.
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