This photo shows RoK President Moon Jae-in (L) meeting with US President Donald Trump at the White House in Washington on May 22, 2018.

Trump met with the Republic of Korea (RoK) President Moon Jae-in at the White House ahead of the historic summit set for June 12 in Singapore.

"There's a chance that it will work out. There's a very substantial chance it won't work out," Trump said at the top of his talks with Moon. "That doesn't mean it won't work out over a period of time. But it may not work out for June 12.

But there's a good chance that we'll have the meeting."

Preparations for the summit continue, but it could be canceled if US conditions aren't met, he added.

DPRK has threatened to pull out of the summit over US demands for "unilateral" denuclearization. The Trump administration maintains that its aim is the complete, verifiable and irreversible dismantlement of DPRK's nuclear program, with no promise of concessions until that process is in motion.

Trump said DPRK's denuclearization must take place, and while the summit may not happen June 12, it could still take place later.

"I do think he's serious. I think he would like to see that happen," Trump said of Kim's willingness to give up his nuclear weapons program.

Secretary of State Mike Pompeo later told reporters the Trump administration was still planning for a June 12 summit, but he declined to predict whether it would actually take place.

Trump's Oval Office remarks were the strongest sign from him yet about the possibility of a delay or cancellation of what would be the first-ever summit between the leaders of the United States and DPRK.

He acknowledged the difficulty of getting DPRK to dismantle the program all at once without offering concessions.

The two Koreas could eventually reunify, he added, but for now, it's unclear whether Moon will meet with Kim again after their first summit on the inter-Korean border last month.

Moon, who took office a year ago, has vowed not to allow war on the peninsula again.