Donald Trump yesterday fired off a verbal salvo at North Korean leader Kim Jong Un, picking on the latter's height and weight as the two leaders kept up their word war.

The US president issued a sarcastic tweet in an apparent response to recent descriptions of him in North Korean media as a "lunatic old man."

"Why would Kim Jong Un insult me by calling me 'old,' when I would NEVER call him 'short and fat'?" Trump tweeted from Hanoi.

Before yesterday's blast, Trump had referred to Kim as a "madman" and "Rocket Man" prompting the latter to deride the US leader as a "mentally deranged dotard."

Trump has played hawk and dove with the North of late denouncing it as a "cruel dictatorship," while offering Kim a diplomatic way out of the crisis over Pyongyang's growing nuclear arsenal.

Trump was a bit conciliatory yesterday.

"Oh well, I try so hard to be his friend - and maybe someday that will happen!" he wrote.