After his curt dismissal over Twitter of talks with North Korea, U.S. President Donald Trump has left many wondering — and some fearing — what his next move against the nuclear-armed nation will be.

On Sunday, Trump called negotiations with Pyongyang a waste of time a day after his own secretary of state acknowledged that the United States is maintaining direct channels of communications with North Korea — even as tensions surge over the isolated nation's nuclear and missile programs.

"I told Rex Tillerson, our wonderful Secretary of State, that he is wasting his time trying to negotiate with Little Rocket Man," Trump wrote on Twitter, using his sarcastic nickname for North Korean leader Kim Jong Un. "Save your energy Rex, we'll do what has to be done!"