President Donald Trump said his planned summit with North Korean leader Kim Jong-un could either fail or bring about the greatest deal for the world.
At a political rally in Pennsylvania, Mr Trump told supporters he believed North Korea wanted to make peace.
But he said he might leave the talks quickly if it didn't look like progress for nuclear disarmament could be made.
In his speech, the US leader warned of tariffs on European cars, and launched his slogan for re-election in 2020.
"Hey, who knows what's going to happen?" said Mr Trump on Saturday at the rally for a Republican congressional candidate. "I may leave leave fast or we may sit down and make the greatest deal for the world."
In his wide-ranging speech, he said he hoped a deal to ease nuclear tensions would happen, particularly to help countries like North Korea.
He also said he believed the North Koreans would honor their commitment not to test any more missiles. Mr Trump told the crowd, "I think they want to make peace, I think it's time."
No date or place has been set for the meeting, despite initial reports it would happen by the end of May.
No sitting US president has ever met a North Korean leader and Mr Trump's decision to accept an invitation from the North Korean leader - relayed by South Korean envoys on Thursday - reportedly took top administration officials by surprise.
Mr Trump tweeted saying a deal was "very much in the making", though the White House said the meeting would not take place unless Pyongyang took "concrete actions".
The US has made "zero concessions" with its sanctions, said Vice-President Mike Pence, following news of the upcoming meeting being agreed. He said he believed the North Korean decision to meet proved the US strategy of isolating North Korea was working.
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