President Donald Trump took to Twitter Sunday morning to tweet foreign policy objectives on North Korea.

“I told Rex Tillerson, our wonderful Secretary of State, that he is wasting his time trying to negotiate with Little Rocket Man. Save your energy Rex, we'll do what has to be done!” said Trump across two tweets.

The president has taken to calling North Korea’s leader, Kim Jong Un, “Rocket Man,” because of the country’s continued testing of missiles.

The tweet comes after Tillerson acknowledged Saturday that the U.S. State Department has had a direct back-channel line of communication with North Korea, despite growing tensions between the U.S. and North Korea.

“We have lines of communication to Pyongyang. We're not in a dark situation, a blackout,” said Tillerson to reporters while on a visit to China. “We have a couple ... three channels open to Pyongyang. We can talk to them, we do talk to them.”

The U.S. technically does not have official diplomatic relations with North Korea, but the U.S. State Department has communicated with North Korea’s through its representatives at the United Nations (U.N.), according to the Associated Press.

“I think the most immediate action that we need is to calm things down. They’re a little overheated right now. And I think we need to calm them down first,” said Tillerson. “Obviously it would help if North Korea would stop firing off missiles. That would calm things down a lot.”

North Korea this year has been aggressively improving its missile and nuclear weapons programs, showcasing is abilities to the world through its continued tests. North Korea tested its sixth and largest nuclear weapon at the beginning of September, claiming that it was a hydrogen bomb. North Korea has also tested two intercontinental ballistic missiles this year that experts believe theoretically have the range to hit the continental U.S.

North Korea is known for their blustery rhetoric often disseminated through its state media wing, but the country has found a verbal sparring partner in Trump. In a September speech to the U. N. General Assembly, Trump called Kim “Rocket Man” again and said that he was on a “suicide mission.” Trump also threatened to “totally destroy North Korea.”