US will be forced to 'totally destroy' North Korea, Trump tells UN

President Donald Trump has warned the US will be forced to "totally destroy" North Korea unless Pyongyang backs down from its nuclear standoff.

"We will have no choice than to totally destroy North Korea,” the president said, if North Korea does not stand down.

Mocking North Korean leader Kim Jong Un, Trump described him as a "rocket man" on a "suicide mission for himself and his regime".

Loud murmurs filled the green-marbled UN General Assembly hall on Tuesday when Trump issued his sternest warning yet to North Korea, whose ballistic missile launches and nuclear tests have rattled the globe.

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

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

Beginning his maiden speech shortly after arriving at UN headquarters in New York, Trump said the world faces destructive threats from rogue states and "terrorists and extremists".

"Rogue regimes not only support terrorists but threaten other nations with the most destructive weapon known to humanity," Trump said, referring to nuclear weapons.

It was Trump's first appearance in the assembly hall, where applause from world leaders is generally muted.

Reading carefully from a script, Trump promised the United States military would soon be the strongest it has ever been.

Trump, who came to power promoting an America First agenda, told world leaders that the United States does not seek to impose its will on other nations and will respect other countries' sovereignty.

"I will defend America's interests above all else," he said.

"But in fulfilling our obligations to other nations we also realise it's in everyone's interest to seek a future where all nations can be sovereign, prosperous and secure."

Earlier Trump tweeted: "Big day at the United Nations - many good things, and some tricky ones, happening. We have a great team."