President Trump defended his saber-rattling with North Korean leader Kim Jong Un by saying it was no longer time to be "nice" to the dictator.

"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," Trump tweeted, using his derogatory nickname for Kim.

Earlier in the day, Trump tweeted he told Secretary of State Rex Tillerson to stop "wasting his time" negotiating with the North Koreans.

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

Trump and Kim have used the media to engage in a war of words that the North Koreans seem to be taking increasingly seriously. Last week, the country's foreign minister said Trump's tweets threatening the lives of North Koreans are tantamount to a declaration of war.

The administration stated the United States is not at war with North Korea.

In his tweet, Trump alluded to attempts to make peace with Kim not working over the last several administrations. Kim has only been in power since his father died in 2011.