President Trump said in a tweet Sunday that he told Secretary of State Rex Tillerson "he is wasting his time trying to negotiate with Little Rocket Man" -- a reference to North Korea's leader.

I told Rex Tillerson, our wonderful Secretary of State, that he is wasting his time trying to negotiate with Little Rocket Man... — Donald J. Trump (@realDonaldTrump) October 1, 2017

Mr. Trump has used the term before, first calling Kim Jong Un "Rocket Man" while delivering remarks at the United Nations General Assembly earlier this month. In his speech to the UNGA, Mr. Trump warned the North Korean regime it risked "total destruction" if it continues its pursuit of nuclear weapons.

He also told the international body that "Rocket Man is on a suicide mission for himself and his regime."

The State Department has since said U.S. diplomats have "open channels" for communicating with the North Koreans, after Secretary Tillerson said during a visit to China Saturday that the U.S. is "probing" talks with North Korea. Tillerson told reporters the U.S. government and North Korean government have "direct communication" about North Korea's nuclear program.

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But the Secretary appeared to walk back those comments later in the day, saying North Korea doesn't appear ready to talk or negotiate.

In another tweet Sunday, Mr. Trump encouraged Tillerson to "save your energy," adding "we'll do what has to be done!"

...Save your energy Rex, we'll do what has to be done! — Donald J. Trump (@realDonaldTrump) October 1, 2017

He added later on Sunday, "Being nice to Rocket Man hasn't worked in 25 years, why would it work now?" referring to President's Clinton, Bush and Obama's "failed" attempts at brokering peaceful negotiations with the regime's prior dictators. He suggested, "I won't fail."