US President Donald Trump deployed his spy chief overnight to sell his snap decision to engage North Korea’s Kim Jong-un in momentous nuclear talks that the president himself predicted would be a “tremendous success” but others warn carry big risks.

CIA director Mike Pompeo portrayed North Korea as buckling under the pressure of US-led international sanctions, and insisted there would be no let-up for the duration of the negotiations.

“Never before have we had the North Koreans in a position where their economy was at such risk, where their leadership was under such pressure,” he said on Fox News Sunday.

“Make no mistake: while these negotiations are going on, there will be no concessions made,” he said.

The US Sunday talk show appearances by Director Pompeo and others served to answer critics who warn that the talks, entered into by an impulsive, inexperienced president, carry high risks.

If they fail, the two nuclear-armed states could then be left with few options short of military confrontation, experts on the years-long impasse with North Korea say.

Director Pompeo suggested that Mr Trump understood the dangers. “The president isn’t doing this for theatre, he is going there to solve a problem.”

Mr Trump used a weekend campaign rally in Pennsylvania steel country to defend his decision to sit down with Kim after months of insult-filled brinkmanship, replete with nuclear threats.

He said the United States had “shown great strength” when tensions were high but the regime’s leaders “want to make peace.” “I think it’s time,” Mr Trump told supporters.

Before boarding his Marine One helicopter for the rally, he told reporters: “I think North Korea is going to go very well, I think we will have tremendous success ... We have a lot of support.

“The promise is they wouldn’t be shooting off missiles in the meantime, and they’re looking to de-nuke. So that’d be great.”

Mr Trump accepted the invitation last Thursday after it was relayed to him in an impromptu White House meeting with the South Korean national security adviser, Chung Eui-yong.

Mr Chung, who had met with Kim previously, told Mr Trump that the North Korean leader had pledged to halt missile and nuclear tests during the negotiations, to discuss denuclearisation and to raise no public objections to scheduled US-South Korean military exercises.

What comes next is unclear.

Deputy press secretary Raj Shah would not rule out a White House summit or Mr Trump going to North Korea for the talks, although he said on US TV that the latter venue was not “highly likely.”

Director Pompeo said “channels are open” but he shed no light on how the United States will proceed or even whether it has heard back from the North Koreans on Mr Trump’s agreement to talk to Kim.