Donald Trump says Rex Tillerson is 'wasting his time' talking to North Korea
President Donald Trump said his chief diplomat is "wasting his time" trying to negotiate with North Korea about its nuclear and missile programmes.
Mr Trump tweeted the advice to US Secretary of State Rex Tillerson: "Save your energy, Rex, we'll do what has to be done!"
The president did not elaborate, but he told the United Nations last month that if the US is "forced to defend itself or its allies, we will have no choice but to totally destroy North Korea".
Mr Tillerson, while in China, said on Saturday the US is maintaining direct channels of communications with North Korea and probing the North's willingness to talk.
He said Washington was "probing" the possibility of more formal talks, and that "lines of communications" in use with Pyongyang.
"Stay tuned," he added.
It is the first confirmation that back channel discussions have been taking place, as Donald Trump and North Korean leader Kim Jong-un's heated rhetoric has sparked fears of military conflict.
But Mr Trump started up again on Sunday, saying he has told Mr Tillerson that "he's wasting his time trying to negotiate with Little Rocket Man", a reference he has coined for North Korean leader Kim Jong-un.
He followed it up several hours later, stating previous presidents Bill Clinton, George W. Bush and Barack Obama had failed when dealing with North Korea and reaffirmed that he would not.
North Korea has in recent months tested long-range missiles that potentially could reach the US, and on September 3 conducted its largest nuclear test explosion to date.