Foreign Minister Julie Bishop has rejected Donald Trump's comments on North Korea after the US President said it was "a waste of time" trying to negotiate with dictator Kim Jong Un.

On Monday, Ms Bishop urged Mr Trump to put maximum political, diplomatic and economic pressure on North Korea to bring it back to the negotiating table.

Mr Trump labelled Kim Jong Un "little rocket man" again on Monday morning.

"I told Rex Tillerson, our wonderful Secretary of State, that he is wasting his time trying to negotiate with Little Rocket Man," Mr Trump wrote on Twitter.