US President Donald Trump made the decision to hold talks with North Korean leader Kim Jong Un himself, US Secretary of State Rex Tillerson said on Friday, but the talks will take "some weeks" to arrange.

Trump has said he was prepared to meet Kim in what would be the first face-to-face encounter between the two countries' leaders and potentially mark a major breakthrough in nuclear tensions with Pyongyang.

"That is a decision the president took himself. I spoke to him very early this morning about that decision and we had a very good conversation," Tillerson told reporters during a visit to the African nation of Djibouti.

"President Trump has said for some time that he was open to talks and he would willingly meet with Kim when conditions were right," the top US diplomat said.

"And I think in the president's judgment that time has arrived now."

He said the United States was surprised at how "forward-leaning" Kim was in his conversations with a visiting South Korean delegation. He said it was the strongest indication to date of Kim's "not just willingness but really his desire for talks".

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