WASHINGTON – President Trump on Sunday said Rex Tillerson is "wasting his time" by trying to negotiate with North Korean leader Kim Jong Un, in a tweet that appeared to undercut his own secretary of State's diplomatic efforts to resolve the nuclear standoff.

“I told Rex Tillerson, our wonderful Secretary of State, that he is wasting his time trying to negotiate with Little Rocket Man,” the president tweeted, using his nickname for North Korea's leader. “Save your energy Rex, we'll do what has to be done!”

Later on Sunday, Trump tweeted: "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."

Trump's tweets about the 33-year-old leader Kim Jong Un came one day after Tillerson acknowledged for the first time that the administration is in “direct contact" with Pyongyang and is seeking a dialogue about its missile and nuclear tests.

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“We are probing, so stay tuned,” Tillerson told reporters in Beijing when asked about a potential dialogue with Kim.

“We ask, ‘Would you like to talk?’ We have lines of communications to Pyongyang — we’re not in a dark situation, a blackout. We have a couple, three channels open to Pyongyang,” he said Saturday.

Yet it’s unclear how successful diplomatic communications will be in curbing North Korea’s nuclear ambitions, since leaders of both countries have been caught in an escalating war of words in recent months.

At the United Nations in September, Trump said the United States may have to “totally destroy” North Korea if the U.S. is forced to defend itself or its allies. The president said Kim is “on a suicide mission for himself and for his regime." In return, Kim called Trump a "mentally deranged dotard" and threatened to "tame (him) with fire."

North Korea, meanwhile, has continued testing missiles and is threatening to test a hydrogen bomb in the Pacific and to shoot down U.S. military aircraft off its coast.

The White House announced Friday that Trump will travel to the region next month. His itinerary includes stops in China, South Korea and Japan.

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Samantha Power, U.S. ambassador to the United Nations under former president Barack Obama, called Trump's tweet about the expected failure of negotiations with North Korea "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?" Power tweeted on Sunday.

After Trump's tweets, State Department spokeswoman Heather Nauert insisted that diplomatic channels with Kim are open – for now.

"They won't be open forever," she said.