President Donald Trump on Sunday said he told Secretary of State Rex Tillerson to “save his energy” when it comes to negotiations with North Korean leader Kim Jong Un, and that the United States will “do what has to be done” instead.

“I told Rex Tillerson, our wonderful Secretary of State, that he is wasting his time trying to negotiate with Little Rocket Man,” Trump tweeted, using his favorite moniker for Kim. “Save your energy Rex, 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

Tillerson on Saturday said the United States has direct channels of communications with North Korea, despite Kim’s and Trump’s publicly escalating rhetoric.

“We have lines of communication to Pyongyang. We’re not in a dark situation, a blackout,” Tillerson said. “We can talk to them, we do talk to them.”

Later Saturday, he said “the whole situation” was “a bit overheated.”

“I think we need to calm them down first,” Tillerson said. “Obviously it would help if North Korea would stop firing off missiles. That would calm things down a lot.”