Donald Trump has an "absolute duty" to prepare for military intervention against North Korea in the face of the mounting nuclear threat posed by the regime, Boris Johnson will say.

The Foreign Secretary will warn that Kim Jong-un, the North Korean leader, is on the verge of acquiring that capability of "make good his threat" to reduce New York to "ashes".

It comes after the US President yesterday said that the US is "totally prepared" for war with North Korea as he increased the US military presence in North Korea. He has previously threatened to unleash "fire and fury".

Mr Johnson, speaking at the annual Chatham House conference in London, will say: "I am afraid that the US President – whoever he or she might be – will have an absolute duty to prepare any option to keep safe not only the American people but all those who have sheltered under the American nuclear umbrella."

However he will urge Mr Trump to show "far-sightedness" and not quit the Iran nuclear deal, arguing it shows the kind of "diplomatic imagination" that could end the North Korea crisis.