North Korean leader Kim Jong-Un Friday mocked Donald Trump as "mentally deranged" -- and warned he will make the US president "pay dearly" for threatening the destruction of his country at the United Nations.

"I will make the man holding the prerogative of the supreme command in the US pay dearly for his speech calling for totally destroying the DPRK," Kim was quoted as saying by state news agency KCNA, referring to North Korea's official name.