US President Donald Trump has said he would never call Kim Jong-Un "short and fat", after months of trading personal insults and threats of war with the North Korean leader.

"Why would Kim Jong-Un insult me by calling me 'old,' when I would NEVER call him 'short and fat?' Oh well, I try so hard to be his friend - and maybe someday that will happen!" Trump tweeted from Hanoi, the latest leg on his marathon tour of Asia.