Donald Trump has hit back at Kim Jong Un after the North Korean dictator called him "deranged" in a bullish statement.
The secretive state's leader had said Mr Trump would "pay dearly" for his threat to "totally destroy" North Korea during the United Nations General Assembly.
The US President responded with an early morning tweet, saying:
Kim Jong Un of North Korea, who is obviously a madman who doesn't mind starving or killing his people, will be tested like never before!
— Donald J. Trump (@realDonaldTrump) September 22, 2017
Mr Kim had said the UN speech - which was boycotted by North Korean diplomats - was "the most ferocious declaration of war in history".
He said North Korea's nuclear programme would continue and that Pyongyang would consider the "highest level of hardline countermeasure in history" against the US.
Describing Mr Trump as "mentally deranged," he added: "After taking office, Trump has rendered the world restless through threats and blackmail against all countries in the world.
"He is unfit to hold the prerogative of supreme command of a country, and he is surely a rogue and a gangster fond of playing with fire, rather than a politician."
United States President Donald Trump speaks during the United Nations General Assembly | Image: Seth Wenig/AP/Press Association Images
Earlier in the week, Mr Trump had used his first speech in front of the UN General Assembly to describe Mr Kim as being a "rocket man on a suicide mission for himself and his regime".
Mr Trump had said: "The United States is ready, willing and able (to act), but hopefully this will not be necessary."
The president also called on member states to work together to isolate Mr Kim until he stops his "hostile" actions, a reference to a series of recent ballistic missile launches and nuclear tests by Pyongyang.
Mr Kim has promised to complete his nuclear programme in the face of strengthening sanctions, saying his state is nearing its goal of "equilibrium" in military force with the US.
On Thursday, Mr Trump signed an executive order to introduce new sanctions against North Korea over its nuclear weapons programme.
The order boosts the US Treasury's ability to clamp down on individuals and companies that finance and facilitate trade with the reclusive state.
President Trump said the measures would "cut off sources of revenue that fund North Korea's efforts to develop the deadliest weapons known to humankind".
It comes as European Union ambassadors agreed draft sanctions against North Korea, including a ban on investments in the country and on EU exports of oil.