President Trump fired back at North Korean leader Kim Jong Un on Friday after Kim stated the U.S. would "pay dearly" for threats Trump made in his speech at the United Nations.

"Kim Jong Un of North Korea, who is obviously a madman who doesn't mind starving or killing his people, will be tested like never before!" Trump tweeted.

Kim Jong Un of North Korea, who is obviously a madman who doesn't mind starving or killing his people, will be tested like never before! — Donald J. Trump (@realDonaldTrump) September 22, 2017

Trump's remarks come in response to a statement made personally by Kim on Thursday in which he rips Trump as "mentally deranged" and likened the U.S. president to a frightened dog.

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"The mentally deranged behavior of the U.S. president openly expressing on the U.N. arena the unethical will to 'totally destroy' a sovereign state, beyond the boundary of threats of regime change or overturn of social system, makes even those with normal thinking think about discretion and composure," Kim said Thursday.

"A frightened dog barks louder," he added.

Trump called Kim "Rocket Man" in his first address to the U.N. General Assembly on Tuesday and used his speech to put the rogue nation and its allies on notice in no uncertain terms after North Korea's latest missile test landed in waters near Japan after flying over the country.

"Rocket Man is on a suicide mission for himself and for his regime,” Trump told the U.N. on Tuesday. “North Korea's missiles and weapons development is a grave threat to peace and security in our world, and it is unacceptable that others financially support this criminal rogue regime."