Washington (CNN) Secretary of State Mike Pompeo said getting North Korea to dismantle its nuclear and missile programs is a "decades long challenge," as President Donald Trump said Monday he remains confident Kim Jong Un will follow through on his pledges to do so.

Pompeo was speaking to troops during an unannounced stop in Afghanistan, days after North Korea blasted his performance during talks in Pyongyang, accusing the US of "gangster-like" behavior.

"Look, this is a decades long challenge, getting the North Koreans to make a fundamental strategic decision, which is that the nuclear weapons that they possess today frankly present a threat to them and not security," Pompeo said.

The State Department said the secretary's mention of 'decades' refers to how long the US has been confronted by the threat of a nuclear weapons program in North Korea and not a timeline for resolving the crisis.

The top US diplomat said North Korea has "for decades told their own people that without nuclear weapons their country was at risk of being attacked by the west, by America, by some other country." The job for the US now, he said, is "to get the entire country to understand that they have that strategically wrong. Chairman Kim told President Trump he understood that. I was there. I saw it."

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