WASHINGTON—President Donald Trump said he didn’t think it was worth pursuing negotiations with North Korean leader Kim Jong Un, a day after his secretary of state revealed the U.S. was in direct contact with Pyongyang.

“I told Rex Tillerson, our wonderful Secretary of State, that he is wasting his time trying to negotiate with Little Rocket Man,” Mr. Trump said on Twitter on Sunday, using a disparaging reference to Mr. Kim. “Save your energy Rex, we’ll do what has to be done!”

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