Watch: Boris Johnson warns of 'catastrophe' if Kim Jong Un realises nuclear ambition

Boris Johnson has warned the situation in North Korea is "very, very serious" after referring to Donald Trump's mockery of "Rocket Man" Kim Jong Un.

The Foreign Secretary said the world faced a catastrophe if North Korea - a "rogue nation" - was able to mount a nuclear warhead on an intercontinental ballistic missile (ICBM).