Prime Minister Malcolm Turnbull has backed US President Donald Trump's fiery language on North Korea, saying "the whole country will be wiped out" and "many, many thousands of innocent people will die" if the regime attacks the United States or its allies.

In his sternest warning yet to North Korea, President Trump told the United Nations General Assembly the United States will be forced to "totally destroy" North Korea unless Pyongyang backs down from its nuclear challenge, and mocked North Korean leader Kim Jong-un as a "rocket man" on a suicide mission.

At President Trump's first appearance at the assembly's annual gathering of world leaders, he emphasised it was against the interest of the entire world for North Korea, which he called a "band of criminals", to obtain missiles and nuclear weapons.

Unless North Korea backed down, he said, "we will have no choice but to totally destroy North Korea."