Kim Jong-un has said remarks by “mentally deranged dotard” US President Donald Trump have convinced him he is right to develop weapons for North Korea.
In an unprecedented personal statement, Mr Kim said Mr Trump would “pay dearly” for a UN speech where he threatened to “totally destroy” the North if the US was forced to defend itself.
Mr Trump responded that the “madman… will be tested like never before”.
The two countries have engaged in ever more heated rhetoric in recent months.
China responded to the war of words, warning that the situation was “complicated and sensitive”.
“All relevant parties should exercise restraint instead of provoking each other,” said Foreign Minister spokesman Lu Kang.
Russia also urged restraint. Kremlin spokesman Dmitry Peskov said Moscow was “deeply concerned by an escalation of tensions”.