Donald Trump let loose on Twitter Sunday with a shoot-from-the-hip volley of posts referring to North Korea's leader Kim Jong-Un as "Rocket Man" - and picturing himself taking out Hillary Clinton with a golf ball.

The US leader spoke Saturday night with South Korea's President Moon Jae-In, pledging joint "steps to strengthen deterrence and defense capabilities and to maximize economic and diplomatic pressure on North Korea," according to the White House.

Mr Trump's own account of the conversation, which kicked off an unbridled salvo of early morning tweets, struck a less diplomatic tone.