US President Donald Trump has followed up his promise to solve "probably all" of the world's problems with the help of China by offering to mediate the intractable dispute in the South China Sea.

Meanwhile, the 71-year-old president has hit out at North Korea's Kim Jong-un (believed to be 33) on Twitter, complaining about being called "old".

"Why would Kim Jong-un insult me by calling me 'old,' when I would NEVER call him 'short and fat?'" Trump tweeted on Sunday.

"Oh well, I try so hard to be his friend – and maybe someday that will happen!"