Kim Jong-un has said remarks by “deranged” US President Donald Trump have convinced him he is right to develop weapons for North Korea.
In a rare personal statement, via state media, the North Korean leader said Mr Trump would “pay dearly” for his recent speech to the UN.
The US president said on Tuesday that if America was forced to defend itself it would “totally destroy” North Korea.
Mr Trump also mockingly called Mr Kim a “rocket man” on a “suicide mission”.
The two countries have been engaging in increasingly heated rhetoric in recent months.
North Korea has been testing missiles at an unprecedented rate, and conducted its sixth nuclear test despite international condemnation.
North Korean Foreign Minister Ri Yong-ho, who had earlier compared Mr Trump’s speech to “the sound of a barking dog”, has warned that Pyongyang could test a hydrogen bomb in the Pacific Ocean in response to the US president’s threat, reported South Korea’s Yonhap news agency.
Mr Kim said in an English statement carried by state news agency KCNA that Mr Trump’s remarks “have convinced me, rather than frightening or stopping me, that the path I chose is correct and that it is the one I have to follow to the last”.
He said “now that Trump has denied the existence of and insulted me and my country in front of the eyes of the world and made the most ferocious declaration of a war in history”, North Korea would consider the “highest level of hard-line countermeasure” to make Mr Trump “pay dearly for his speech”.
He ended by saying he would “surely and definitely tame the mentally deranged US dotard with fire”, employing a rarely used word for an old person.
Mr Trump on Thursday signed a new order boosting sanctions against North Korea, where the US treasury would target firms and financial institutions doing business with Pyongyang.
He said: “For much too long North Korea has been allowed to abuse the international financial system to facilitate funding for its nuclear weapons and missile programs.”
The UN Security Council had approved new rounds of sanctions earlier this month aimed of starving North Korea of fuel and income, which were in response to Pyongyang’s much-condemned sixth nuclear test on 3 September.
North Korea was one of the top issues dominating the agenda at the UN General Assembly in New York, where several speakers called for a de-escalation of tensions.
Russian Foreign Minister Sergei Lavrov warned that “military hysteria” over the nuclear testing would lead to “disaster”, while China’s Foreign Minister Wang Yi told Pyongyang not to go in a “dangerous direction”.
Mr Wang told the UN on Thursday that there should be no new nuclear weapons on the Korean peninsula “whether it is in the North or the South”.