US President Donald Trump has told his Secretary of State (foreign minister) not to waste his time trying to negotiate with North Korean leader Kim Jong Un.

He fired off two ominous tweets saying he told his top USdiplomat Rex Tillerson not to waste his time trying to negotiate with North Korean leader Kim Jong Un.

Adding: "Save your energy Rex, we'll do what has to be done!"

The tweets came  a day after Tillerson told reporters that the United States was directly communicating with North Korea on its nuclear and missile programmes, suggesting that there may be a way forward without escalating threats of open conflict between the two countries.

The disclosure by Tillerson during a trip to China was the first time he has spoken to such an extent about US outreach to North Korea.

The president has shifted back and forth between direct personal attacks on the North Korean leader and a willingness to negotiate. After announcing new U.S. sanctions last month on North Korea, he also acknowledged diplomacy was still possible.

But Trump's tweets likely making it harder for the US to establish the dialogue Tillerson was seeking.