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 (19) that if America was forced to defend itself it would "totally destroy" North Korea.
Trump also mockingly called Kim a "rocket man" on a "suicide mission".
The two countries have been engaging in increasingly heated rhetoric in recent months.