Donald Trump labels Kim Jong-un 'Rocket Man' over growing nuclear threat in antagonistic tweet

US President Donald Trump has called Kim Jong-un "Rocket Man". Photo / AP

Donald Trump has taunted North Korean leader Kim Jong-un in a tweet about a conversation with the South Korean president.

The US leader spoke Saturday night with Moon Jae-in, pledging joint "steps to strengthen deterrence and defence capabilities and to maximise economic and diplomatic pressure on North Korea," according to the White House.

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

"I spoke with President Moon of South Korea last night. Asked him how Rocket Man is doing. Long gas lines forming in North Korea. Too bad!" the president wrote to his 38 million followers.

He then launched into half a dozen retweets of internet memes celebrating his Make America Great Again campaign theme - topping it off with a GIF montage of himself taking a swing at a golf ball and knocking out his erstwhile Democratic rival Hillary Clinton.

The Republican leader is set to address the United Nations General Assembly in New York for the first time on Tuesday, focusing on North Korea and Iran's "destabilising behaviour" in the Middle East.

UN has run out of options

The US ambassador to the United Nations Nikki Haley has said the UN Security Council has run out of options on containing North Korea's nuclear program and the US may have to turn the matter over to the Pentagon.

"We have pretty much exhausted all the things that we can do at the Security Council at this point," Haley told CNN's State of the Union on yesterday, adding that she was perfectly happy to hand the matter to Defence Secretary James Mattis.

"We're trying every other possibility that we have but there's a whole lot of military options on the table."

Haley also defended President Trump's use of Twitter after he was scolded by the British prime minister over his London attack tweet.

"You have to understand the president can communicate in any form that he wishes. And if he uses Twitter, I think what you saw was a reaction to the fact that this was terrible and we cannot let terrorist attacks become part of our normal day," she said.

"And when you see a number of these, it makes the president more and more assured that he has to do everything he can to keep Americans safe."