Donald Trump has called Kim Jong-un a "madman who doesn't mind starving or killing his people", declaring the North Korean leader will be "tested like never before".
The declaration came in a tweet and followed a North Korean threat to detonate a nuclear weapon over the Pacific Ocean.
Mr Trump's fiery speech at the United Nations earlier this week has served to ramp up the tensions between the two countries.
The tweet was sent shortly before 4am in Washington, DC time.
Shortly after making the announcement, Mr Trump re-tweeted a number of messages complimenting his controversial UN performance.
Echoing previous comments, Secretary of State Rex Tillerson then said diplomatic efforts with North Korea would continue but the US was open to using military options.
Relations between the US and North Korea have significantly deteriorated in recent months, with the two leaders stooping to trading personal insults.
Earlier, Mr Kim called Mr Trump a "mentally deranged US dotard', meaning a senile elderly person. He threatened to "tame" Mr Trump with "fire".
Conversely, the US President has taken to calling Mr Kim 'Rocket Man', apparently after his tendency to test fire missiles against international rules.
Mr Trump has used much more severe rhetoric than his predecessors against the rogue state. In August, he threatened "fire and fury like the world has never seen" against North Korea, if it endangered US interests.
China, North Korea's closest ally, has called for restraint between the two parties.
