Trump to Rex: Stop negotiating with Kim Jong-un, he's on a suicide mission

United States President Donald Trump took to Twitter to advise Secretary of State Rex Tillerson to stop negotiating with North Korea's Rocket Man because the US will do what has to be done.

"I told Rex Tillerson, our wonderful Secretary of State, that he is wasting his time trying to negotiate with Little Rocket Man," Trump tweeted on Sunday.

Trump had first termed North Korean leader Kim Jong-Un as "Rocket Man who is on suicide mission" at United Nations General Assembly session last month.

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

President's comments come a day after Rex Tillerson disclosed the United States has direct lines of communications with Pyongyang. "We ask, 'Would you like to talk?' We have lines of communications to Pyongyang - we're not in a dark situation, a blackout," Secretary of State Rex W. Tillerson said.

"We have a couple, three channels open to Pyongyang," he added."We've made it clear that we hope to resolve this through talks," he said, emphasizing the principal objective "is a peaceful resolution."

"I think the most immediate action that we need is to calm things down," Tillerson added. "They're a little overheated right now, and I think we need to calm them down first."

After oil sanctions, North Korea's leaders have hinted that more ballistic missile and nuclear weapons tests will take place.