Video report by ITV News Security Editor Rohit Kachroo
Kim Jong-un has said "deranged" US President Donald Trump will "pay dearly" for his threat to "totally destroy" North Korea.
The North Korean leader also described Mr Trump as "unfit to hold the prerogative of supreme command of a country", adding that he is "a rogue and a gangster fond of playing with fire."
The statement issued by the North's official Korean Central News Agency comes after Mr Trump mocked Kim as a "rocket man" on a "suicide mission" during a combative speech at the UN General Assembly on Tuesday.
The president threatened to "totally destroy" North Korea if the United States is "forced to defend itself or its allies".
Reacting to the speech, Kim said Mr Trump's remarks have convinced him "that the path I chose is correct and that it is the one I have to follow to the last."
He added that he is "thinking hard" about his response, but the president will "pay dearly for his speech calling for totally destroying" North Korea.
North Korea 'could test hydrogen bomb in Pacific'
In a separate development, South Korean media reported on Friday that the North Korean foreign minister has said his country may test a hydrogen bomb in the Pacific Ocean.
According to South Korea's Yonhap news agency, Ri Yong Ho told reporters at the UN gathering in New York that the test in response to Mr Trump's comments "could be the most powerful detonation of an H-bomb in the Pacific."
Ri reportedly added: "We have no idea about what actions could be taken as it will be ordered by leader Kim Jong-un."
In recent months, North Korea has launched a pair of intercontinental missiles believed capable of striking the continental United States and another pair that soared over Japanese territory.
Earlier this month, North Korea conducted its most powerful nuclear test to date, drawing stiffer UN sanctions.
On Thursday, Mr Trump announced new sanctions against North Korea which help the US target people, companies and banks financing and facilitating trade with North Korea.
He also praised China for reportedly ordering banks to stop doing business with Pyongyang.
Mr Trump said his new executive order would disrupt other trade avenues for North Korea in efforts to halt its nuclear weapons programme.