US President Donald Trump warned on Tuesday that the United States will be forced to 'totally destroy' North Korea unless Pyongyang backs down from its nuclear standoff, mocking North Korean leader Kim Jong Un as a 'rocket man' on a suicide mission.

In a rambling 41-minute speech, Trump criticised socialism, attacked the UN's use of its funds and said the US was paying too much towards it, and praised the importance of patriotism. He condemned the nuclear deal painstakingly struck by the Obama administration as an 'embarrassment'.

His words about North Korea, though, drew the strongest reaction, with loud murmurs filling the green-marbled UN General Assembly hall.

Trump issued his sternest warning yet to North Korea, whose ballistic missile launches and nuclear tests have rattled the globe. He spoke of the death of Otto Warmbier, the American student who died after being returned to the US after a period of imprisonment in North Korea. Trump said the 'entire world' was threatened by the regime.

Unless North Korea backs down, he said, 'We will have no choice than to totally destroy North Korea.'

He said: 'Rocket man is on a suicide mission for himself and his regime.'

He urged United Nations member states to work together to isolate the Kim government until it ceases its 'hostile' behaviour.

A junior North Korean diplomat remained in the delegation's front-row seat for Trump's speech, the North Korean UN mission said.

Additional reporting by Reuters.