Story highlights Tillerson said Saturday he was trying to calm things down

Trump took to Twitter to say the secretary of state should not waste the time

Washington (CNN) President Donald Trump on Sunday again mocked North Korean leader Kim Jong Un and said Secretary of State Rex Tillerson should not bother trying to negotiate with him in an effort to stop the country's development of nuclear weapons.

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

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

...Save your energy Rex, we'll do what has to be done! — Donald J. Trump (@realDonaldTrump) October 1, 2017

Later Sunday afternoon, Trump added: "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."

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. — Donald J. Trump (@realDonaldTrump) October 1, 2017

Kim was officially declared leader of North Korea following the death of his father, Kim Jong Il, in 2011.