US President Donald Trump has vowed to "totally destroy" North Korea unless it backs down from its weapons programs, mocking Pyongyang's leader Kim Jong-un as a "rocket man on a suicide mission".

Key points: His North Korea comments shocked the forum that earlier pushed for statesmanship

His North Korea comments shocked the forum that earlier pushed for statesmanship In his 41-minute speech Mr Trump also touched on Iran and Islamic extremism

In his 41-minute speech Mr Trump also touched on Iran and Islamic extremism Mr Trump repeated his "America First" vision, said he would defend US "above all else"

Mr Trump repeated his "America First" vision, said he would defend US "above all else" Read the full text of Donald Trump's speech

The comments, in Mr Trump's first speech to the UN General Assembly, were his most direct threat to attack North Korea for its hostile activities that have included launching ballistic missiles over Japan and conducting underground nuclear tests.

His comments rattled the world leaders gathered before him in the green-marbled hall, where just minutes earlier, UN Secretary-General Antonio Guterres had appealed for statesmanship and diplomacy.

"The United States has great strength and patience, but if it is forced to defend itself or its allies, we will have no choice but to totally destroy North Korea," Mr Trump said.

As loud, startled murmurs filled the hall, Mr Trump doubled down, referring to Mr Kim by nickname "rocket man" — which he first used on Sunday in a Twitter post — despite the international forum and gravity of the situation.

"Rocket man is on a suicide mission for himself and his regime."

One man in the audience covered his face with his hands shortly after Mr Trump made his "totally destroy" comment, while Swedish Foreign Minister Margot Wallstrom crossed her arms.

North Korea's mission to the United Nations — who had a junior diplomat in the front row — did not immediately respond to a request for comment on Mr Trump's remarks.

Mr Trump's sabre-rattling rhetoric was in contrast to the comments of some of his own administration who have stated their preference for a diplomatic solution.

Shortly before Mr Trump's speech, Mr Guterres appealed from the General Assembly lectern for statesmanship to avoid war with North Korea.

"We must not sleepwalk our way into war," he said.

Later, in what may have been a veiled prod at China, Pyongyang's major trading partner, Mr Trump said: "It is an outrage that some nations would not only trade with such a regime but would arm, supply and financially support a country that imperils the world with nuclear conflict."

Turnbull says Trump 'stating reality'

Prime Minister Malcolm Turnbull described Mr Trump's remarks as "stating the reality".

"It is very important that North Korea understands that if it attacks the US or the Allies, the United States will respond, and respond in a way that would end the North Korean regime," Mr Turnbull said.

The PM said the North Korean situation was one of increasing gravity and risk, and called on the United Nations to increase sanctions "so that it can be brought to its senses without conflict".

When pressed on whether he feared an attack, Mr Turnbull said he did not think the North Korean leader would "commit suicide".

"If he attacks the US, that is a suicide note for his regime," he said.

"It is a disaster because many, many thousands will die. Many, many thousands of innocent people will die."

'Major portions of the world are going to hell': Trump

Mr Trump addressed 193 diplomats at the UN General Assembly's New York headquarters. ( AP: Seth Wenig )

In his thunderous 41-minute speech, Mr Trump also took aim at Iran's nuclear ambitions and regional influence, Venezuela's collapsing democracy, and the threat of Islamist extremists. He also criticised the Cuban Government.

"Major portions of the world are in conflict and some in fact are going to hell," he said.

Turning to Iran, Mr Trump called the 2015 nuclear deal negotiated by his predecessor Barack Obama an embarrassment and hinted that he may not recertify the agreement when it came up for a mid-October deadline.

"I don't think you've heard the last of it," he said, while calling Iran an "economically depleted rogue state" that exported violence.

Tehran was quick to slam Mr Trump's "mean and pointless" remarks on Iran during his UN address.

The semi-official ISNA news agency quoted Foreign Minister Mohammad Javad Zarif as saying Mr Trump's "impudent and ignorant" remarks are divorced from reality, and ignored Iran's role in fighting terrorism.

But all in all, Tuesday's speech marked Mr Trump's latest attempt to lay out his "America First" vision for a US foreign policy aimed at downgrading global bureaucracies, basing alliances on shared interests, and steering Washington away from nation-building exercises abroad.

Mr Trump, who entered the White House eight months ago, told the 193-member global body that the United States did not seek to impose its will on other nations and would 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."

Sorry, this video has expired A US Air Force B-1B bomber, F-35B stealth fighter jets, and South Korean F-15K fighter jets fly over the peninsula.

Reuters/ABC