US President Donald Trump is exchanging schoolyard taunts with North Korea’s Kim Jong-un.

In a response to North Korea branding Trump’s speech in South Korea as “reckless remarks by an old lunatic”, Trump tweeted from Hanoi on Sunday morning: “Why would Kim Jong-un insult me by calling me ‘old,’ when I would NEVER call him ‘short and fat?”‘

Trump goes on to say sarcastically, “Oh well, I try so hard to be his friend – and maybe someday that will happen!”

Trump has been working to rally global pressure against North Korea’s nuclear weapons program on a trip to Asia – a verbal offensive that included a stern speech delivered in South Korea’s National Assembly on Tuesday, in which he said:

“Do not underestimate us. And do not try us … The weapons you’re acquiring are not making you safer, they are putting your regime in grave danger. Every step you take down this dark path increases the peril you face.”

Kim Jung-un: branded Donald Trump an “old lunatic”.

On Saturday, Kim’s government responded by accusing Trump of trying to demonise North Korea, keeping it apart from the international community and undermining its government.

“Reckless remarks by an old lunatic like Trump will never scare us or stop our advance,” the North’s Foreign Ministry said in a statement.

“On the contrary, all this makes us more sure that our choice to promote economic construction at the same time as building up our nuclear force is all the more righteous, and it pushes us to speed up the effort to complete our nuclear force.”

North Korea is not known to have tested any of its missiles or nuclear devices since September 15, a relative lull after a brisk series of tests earlier this year.