UNITED NATIONS • North Korea and Iran will dominate this week's gathering of world leaders at the United Nations, where US President Donald Trump will be firmly in the spotlight as he continues to upend global diplomacy.

After warming up to North Korean leader Kim Jong Un and ditching the Iran nuclear deal, the unpredictable Trump takes the podium tomorrow to face foes and increasingly uneasy allies at the UN General Assembly.

On Wednesday, he will for the first time chair a Security Council meeting on non-proliferation and weapons of mass destruction that will focus heavily on Iran - likely triggering a clash with other big powers.

"It will be the most watched Security Council meeting ever," US Ambassador Nikki Haley said of Mr Trump's first time wielding the gavel.

The diplomatic gathering will take stock of the thaw in relations between North and South Korea, and ground-breaking US-North Korea moves to address the threat from Pyongyang's nuclear and ballistic missile programmes.

Last year, world leaders shuddered as Mr Trump threatened to totally destroy North Korea and belittled Kim as "Rocket Man on a suicide mission".

An exchange of insults ensued, with Mr Kim calling out the "mentally deranged US dotard".

Mr Trump's address to the assembly will be the "polar opposite of what we heard last year", said Ms Suzanne DiMaggio, an expert on North Korea and Iran at the Carnegie Endowment for International Peace.

The President will tout his face-to-face with Mr Kim as a major diplomatic win but "he should think twice if he plans to repeat his claim that North Korea is no longer a nuclear threat", she said.

Despite the Trump-Kim landmark summit in Singapore in June, there has been little concrete progress on denuclearisation.

North Korean Foreign Minister Ri Yong Ho has been invited by US Secretary of State Mike Pompeo for talks on the sidelines of the assembly meeting. Mr Ri is scheduled to deliver his address on Sept 29.

During key meetings, South Korean President Moon Jae-in, who left for New York yesterday, will encourage Mr Trump to press on with the rapprochement.

Mr Moon will likely seek to broker a second US-North Korea summit, reported Yonhap news agency.

The South Korean leader will explain the outcome of the latest inter-Korean summit before the entire world when he addresses the United Nations General Assembly, said Mr Nam Gwan-pyo, a senior director from the presidential National Security Office.

Mr Moon said last week after his third summit with Mr Kim that he had additional messages for Mr Trump that come directly from the North Korean leader.

"Among what we discussed, there are items that we did not include in the joint declaration," he told a nationally televised press conference after his return from North Korea last Thursday.

"I plan to deliver such messages in detail to the US side."

Ties between Teheran and Washington, however, have been strained since the Trump administration pulled out from the landmark 2015 Iranian nuclear deal in May and re-imposed sanctions on the Islamic republic.

Iranian President Hassan Rouhani will address the fallout from the US decision to abandon the nuclear deal when he takes the podium shortly after Mr Trump tomorrow.

European countries, along with Russia and China, are still working to salvage the accord and will use the council meeting chaired by Mr Trump to defend what they consider as a milestone in non-proliferation.

"The members of the Security Council are not going to take kindly to being lectured by President Trump on the subject of Iran," said Ms DiMaggio.

"These very countries, which include our closest allies, are now facing US sanctions as they scramble to save the agreement."

About 130 heads of state and government are turning up for the six-day marathon of speeches and meetings on tackling a long list of issues, from climate change to poverty.

Russia and China will be represented by their foreign ministers.