President Donald Trump toughened his rhetoric against North Korea on Sunday, declaring that attempts to negotiate with the regime were "waste," and the U.S. was prepared to "do what has to be done" as tensions escalate between both countries.

Over the weekend, Secretary of State Rex Tillerson said that the U.S. had opened a direct channel of communication with Pyongyang, which has tested rockets with increasing regularity. Tillerson has said there were at least "a couple" of channels through which the two parties were attempting to negotiate, The Wall Street Journal reported.

However, Trump on Sunday effectively undercut his Secretary of State's diplomacy by declaring negotiations to be all but fruitless. In a series of posts on Twitter, Trump told Tillerson to "save his energy," suggesting the U.S. was prepared to escalate its heated rhetoric to potential military action.

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Later in the day, State Department spokesperson Heather Nauert underscored the U.S.'s resolve by saying Pyongyang would not be allowed to deploy nuclear weapons. The choice of diplomacy or force "is up to the regime," she said on Twitter.

Tillerson told reporters in Beijing last week that "we are probing. Stay tuned," The Journal reported.

"We ask, 'Would you like to talk?' We have lines of communication to Pyongyang. We're not in a dark situation, a blackout," the secretary of state added.

For his part, Trump has adopted a tougher stance against Pyongyang, which culminated last month at a speech at the United Nations. The president has taken to calling Kim "Rocket Man," in light of his insistence on firing rockets, a moniker he used at the world body when he said the U.S. would "totally destroy" North Korea, if provoked.

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