Donald Trump has cast doubt over whether his highly-anticipated meeting with Kim Jong-un, the North Korean leader, will go ahead next month.
"It may not work out for June 12," Mr Trump said. "If it does not happen, maybe it will happen later."
Mr Trump was holding talks with South Korean President Moon Jae-in at the White House, which has been seen as an effort to salvage the landmark meeting.
The US president added that he does believe the North Korean leader is willing to give up nuclear weapons, amid fears about apparent North Korean backpedaling.
"I do think he is serious. I think he is absolutely very serious," Mr Trump said.
"He will be extremely happy" Mr Trump said of Kim, if the deal works out. "He will be very happy."
Today, it was my great honor to welcome President Moon Jae-in of the Republic of Korea to the @WhiteHouse!🇺🇸🇰🇷 pic.twitter.com/yvOxNiA1DM
— Donald J. Trump (@realDonaldTrump) May 22, 2018
Mr Trump went on: "You never know about deals…I’ve made a lot of deals. You never really know."
"You’ll know soon," he added, doing little to quell speculation about the wavering prospects of a historic first summit.
The president declined again to say whether he had yet spoken personally to Kim.
He added that North Koreans were "hardworking, great people".
North Korea’s willingness to denuclearise – the very basis for the talks – was recently plunged into doubt when the rogue state denounced US demands for "unilateral nuclear abandonment."
Pyongyang also canceled at the last minute a high-level meeting with the South in protest over joint military drills between Seoul and Washington.
Mr Trump suggested that Kim’s apparent about-face may have been at the behest of Chinese leader Xi Jinping, after their meeting earlier this month.
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"There was a difference when Kim Jong Un left China the second time," Trump said. "There was a different attitude after that meeting and I was a little surprised.
Mr Trump described President Xi as a "world class poker player", as he reiterated his suggestion that Kim’s recent meetings with the Chinese President had influenced him to harden his stance.
"I think things changed after that meeting so I can’t say that I am happy about it," Mr Trump said.
The US president struck a conciliatory note in a lengthy Q&A with journalists at the White House as he again raised the prospect of a windfall for Kim if he co-operates with the US.
"He will be extremely happy" if the deal works out, Mr Trump said.
Asked whether he could guarantee Kim’s safety, Mr Trump said: "He will be safe, he will be happy, his country will be rich. Look at South Korea – don’t forget, we helped South Korea – and it is one of the most incredible countries in terms of what they do.
"I think that he (Kim) will be extremely happy if things work out. Honestly I think if it doesn’t work out he can’t be happy. "
President Moon, who was instrumental in convincing the two sides to sit down and talk, was at pains to stress the concrete results of diplomacy so far – most notably the release of three US citizens held captive in North Korea.
Mr Trump praised his guest, but also indicated that a separate, second round of high-level talks between Mr Moon and Kim may also be on ice. "Right now, he doesn’t know whether or not he has a meeting," Mr Trump said of the South Korean president.
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