President Donald Trump said that he told Secretary of State Rex Tillerson, America's top diplomat, that he's "wasting his time trying to negotiate" with North Korean leader Kim Jong Un, who Trump again referred to as "Little Rocket Man."

"Save your energy Rex, we'll do what has to be done!" Trump tweeted today.

Trump's words were met with criticism from former officials from the Barack Obama White House. Samantha Power, former U.S. Ambassador to the United Nations, tweeted, "This is life-or-death presidential malpractice. How could any diplomat (or human) tolerate being treated as Tillerson is? How could any other country take seriously the assurance that America seeks a peaceful solution?"

Four hours after his initial comments, Trump appeared to defend his words, saying, "Being nice to Rocket Man hasn't worked in 25 years, why would it work now? Clinton failed, Bush failed, and Obama failed. I won't fail." (Kim Jong Un came to power six years ago, following the death of his father, Kim Jong Il.)

The online exchange came after Tillerson said Saturday that the U.S. had "lines of communication" to the North Korean government. State Department spokesperson Heather Nauert later added the U.S. has "several open channels" to North Korean officials, but they have shown "no indication that they are interested in or are ready for talks regarding denuclearization."

North Korean officials are reportedly struggling to understand Trump, and a top official said Monday that his government had interpreted another of Trump's tweets as a declaration of war. The White House said later that such an interpretation was "absurd."

Related: White House Says Trump Didn't Declare War on North Korea

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