(CNN) — President Donald Trump cast doubt on his planned summit with North Korean leader Kim Jong Un will occur on schedule, saying Tuesday the talks “may not work out for June 12.”

He was speaking alongside his South Korean counterpart, who is in Washington to shore up confidence ahead of the historic meeting.

Trump said preparations were “moving along” for the diplomatic encounter with Kim, but suggested there may not be enough time for the two sides to agree on the parameters for talks.

“If it doesn’t happen, maybe it will happen later,” he said.

In his meetings with South Korean President Moon Jae-in, Trump hoped to gain clarity on North Korea’s nuclear intentions.

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, scheduled for June 12 in Singapore, will proceed. White House aides have grown pessimistic in recent days that the talks will occur, even as Trump has shown no signs he’s ready to withdraw.

Moon, who has urged a diplomatic path in the belief it could forestall war, hopes 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.

“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.”

The two leaders were set to meet in the Oval Office at noon before joining a larger working lunch with aides. There was no joint news conference on the schedule, and Moon was only expected to be at the White House for roughly two hours.

He was expected to 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.

Secretary of State Mike 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.

“Plans continue to go forward for a summit. We remain open to it, we remain hopeful,” Vice President Mike Pence said on Fox Radio Tuesday. “But let me very clear: nothing has changed about the policy of the United States of America. There have been no concessions offered and none given.”

The-CNN-Wire™ & © 2018 Cable News Network, Inc., a Time Warner Company. All rights reserved.

Watch & Listen LIVE