(CNN) Former Secretary of State Condoleezza Rice says she thinks North Korean leader Kim Jong Un is "turning out, much more so than I thought, to be actually pretty clever."

Rice, who served under former President George W. Bush, made the remarks in response to a question about President Donald Trump's tweets on North Korea's nuclear program in a wide-ranging conversation with David Axelrod on "The Axe Files" Saturday night on CNN.

"The approach to the South Koreans was clever; the decision to go to the Olympics is clever," Rice said referring to Kim's overtures on meeting with South Korea to ease military tensions between the two countries. "I think he is more isolated than his father was. Perhaps certainly more reckless. I wonder sometimes if he really believes it when he says, 'I can destroy the United States,' because anybody who tells (him) something he doesn't want to hear seems to get killed."

And that "is a real disincentive to tell him the truth," Rice added.

Rice also discussed Trump's tweets on the nuclear threat from Pyongyang, including one comparing the size of his his "nuclear button" to Kim's, saying she doesn't think such tweets are helpful.

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