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Kim Jong-Un has called Donald Trump's UN speech 'a declaration of war' and branded the US president 'mentally deranged' in a rare address.

The North Korean leader has hardly been heard speaking on camera before - if ever - but has made a withering speech condemning Trump on camera.

Kim has threatened the "highest level of hard line counter measure in history", in response to Trump warning the US could destroy his country.

During his statement, Kim said the American leader should 'be considerate of whom he speaks' when making a speech in front of the world.

"His remarks remind me of such words as 'political layman' and political heretic' which were in vogue in reference to Trump during his presidential election campaign," Kim 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 that he would destroy the DPRK, we will consider with seriousness exercising of a corresponding highest level of hard line counter measure in history."

(Image: KCNA)

"His 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," Kim said in the statement carried by the North's official KCNA news agency.

"As a man representing the DPRK and on behalf of the dignity and honor of my state and people and on my own, I will make the man holding the prerogative of the supreme command in the US pay dearly for his speech," Kim said, referring to the North by its official name, the Democratic People's Republic of Korea.

The inflammatory speech comes after Boris Johnson warned the world faces a “catastrophe that it hasn’t seen before” if Kim Jong Un realises his nuclear ambitions.

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The Foreign Secretary warned the situation in North Korea, - a “rogue nation” was “very, very serious.”

In a Facebook video, Johnson referenced Trump mocking the North Korean leader as “rocket man,” yesterday.

The President told the UN "Rocket Man is on a suicide mission for himself and his regime" and added that if the US was "forced to defend itself or its allies, we will have no choice but to totally destroy North Korea".

(Image: AFP)

Johnson joked: “Elton John is getting a place in Donald Trump’s speeches.”

But he added: “Of course it's very, very serious. The world faces a catastrophe that it hasn't seen before, that is a rogue nation being able to use an ICBM (Inter-continental Ballistic Missile) equipped with a thermonuclear hydrogen warhead.

"That is very, very dangerous, something we have got to stop.

"The key to it, we think, is to work together with all our friends and partners here in the UN, get the Chinese to put more economic pressure on Pyongyang."

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(Image: REUTERS)

At the summit, Mr Johnson met Chinese foreign minister Wang Yi and urged him to keep up the pressure on North Korea.

The Foreign Secretary told the Guardian: "No one in their right minds wants to see the US driven to use its military options - I do not see any good military options - and that is why all the pressure has to be on the Chinese to tighten the sanctions on oil.

"That is the jugular of the North Korean economy."