A showdown looms at the United Nations on Tuesday as US President Donald Trump and Iranian leader Hassan Rouhani are set to square off during the world's biggest diplomatic gathering.

On the opening day of the General Assembly debate, Mr Trump and Mr Rouhani, Iranian President, will take their turn at the podium four months after the United States ditched a nuclear deal with Tehran.

The five remaining parties to the agreement – Britain, China, France, Germany and Russia – announced on Monday that they plan to keep business ties running with Iran, staring down Washington's move to impose sanctions.

Eyeing his second summit with North Korean leader Kim Jong-un, Mr Trump will tout his diplomacy with Pyongyang as a win, even though the North has taken little concrete action to dismantle its missile and nuclear programmes.

Mr Trump withdrew from the nuclear deal in May, to the dismay of European allies, Russia and China, which had invested years in negotiations to achieve a milestone agreement on keeping Iran's nuclear ambitions in check.

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In his address, Mr Rouhani will stress that Iran continues to stick to the 2015 deal and will likely portray the US as a pariah for breaking its international commitments.

Mr Trump used his UN address last year to say the nuclear deal was "an embarrassment", signalling that the US was ready to walk away from the agreement.

After its exit, the US maintains it is seeking to ramp up pressure on Iran which it accuses of sowing chaos in Iraq, Syria, Yemen and Lebanon.

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UNGA in pictures:

Egypt's President El Sisi reaches to shake hands with US President Donald Trump. Reuters United Nations Secretary-General Antonio Guterres speaks at the Nelson Mandela Peace Summit during the 73rd session of the General Assembly of the United Nations at United Nations Headquarters in New York. EPA France's President Emmanuel Macron reaches out to shake hands as he holds bilateral meeting with US President Donald Trump. Reuters Korean K-Pop group BTS speaks on the floor of the Trusteeship Council during the 73rd session of the General Assembly of the United Nations at United Nations Headquarters in New York. EPA White House senior adviser Ivanka Trump listens as she sits beside US Vice President Mike Pence, Secretary of State Mike Pompeo, National Security Adviser John Bolton and White House Press Secretary Sarah Huckabee Sanders during US President Donald Trump's bilateral meeting with South Korean President Moon Jae-in. Reuters Flags fly above diplomatic vehicles on the first day of high-level meeting at United Nations headquarters. EPA Turkish president, Recep Tayyip Erdogan, left, arrives to meet with UN Secretary General Antonio Guterres, right, at United Nations headquarters in New York. EPA US President Donald Trump reacts during a bilateral meeting with France's President Emmanuel Macron. Reuters France's President Emmanuel Macron speaks as he holds a bilateral meeting with US President Donald Trump. Reuters Egypt's President Abdel Fattah El Sisi tucks away a cloth after wiping his face during a bilateral meeting with US President Donald Trump on the sidelines. Reuters Canadian Prime Minister Justin Trudeau, left, shakes hands with Microsoft founder Bill Gates at the High-level Meeting on Financing the 2030 Agenda for Sustainable Development. EPA Canadian Prime Minister Justin Trudeau gestures during the High-level Meeting on Financing the 2030 Agenda for Sustainable Development. EPA Managing Director and Chairwoman of the International Monetary Fund Christine Lagarde, left, speaks with Larry Fink, the chairman and CEO of BlackRock, an American multinational investment management corporation before the High-level Meeting on Financing the 2030 Agenda for Sustainable Development at United Nations headquarters. EPA Managing Director and Chairwoman of the International Monetary Fund Christine Lagarde, left, speaks with UN Secretary General Antonio Guterres before the High-level Meeting on Financing the 2030 Agenda for Sustainable Development at United Nations headquarters in New York. EPA Queen Masenate Mohato Seeiso, left, of Lesotho and Crown Prince Haakon of Norway on the floor of the Trusteeship Council during the 73rd session of the General Assembly of the United Nations at United Nations Headquarters in New York. EPA Peruvian President Martin Vizcarra speaks during an interview with Reuters on the sidelines of the United Nations General Assembly in New York. Reuters Peruvian President Martin Vizcarra speaks during an interview with Reuters on the sidelines of the United Nations General Assembly in New York. Reuters Larry Fink, the chairman and CEO of BlackRock, an American multinational investment management corporation speaks at the High-level Meeting on Financing the 2030 Agenda for Sustainable Development at United Nations headquarters. EPA Clarke Gayford, partner of New Zealand Prime Minister Jacinda Ardern, carries their daughter Neve after listening to Ardern speak at the Nelson Mandela Peace Summit during the 73rd session of the General Assembly of the United Nations at United Nations Headquarters. EPA Members of the Korean K-Pop group BTS attend a meeting at the U.N. high level event regarding youth during the 73rd session of the United Nations General Assembly. AP Photo Bashar al-Ja'afari, second right, Syria's Ambassador to the United Nations, talks with Ibrahim al-Eshaiker al-Jaafari, right, Iraq's Foreign Minister, on the sidelines of the Nelson Mandela Peace Summit during the 73rd session of the General Assembly of the United Nations at United Nations Headquarters in New York. EPA

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"As I have said repeatedly, regime change in Iran is not the administration's policy," Mr Trump's national security adviser John Bolton said on Monday.

"We've imposed very stringent sanctions on Iran, more are coming, and what we expect from Iran is massive changes in their behaviour."

After a meeting on Monday, EU Foreign Policy Chief Federica Mogherini announced a new legal entity would be set up to preserve oil and other business links with Iran.

"This will mean that EU member-states will set up a legal entity to facilitate legitimate financial transactions with Iran and this will allow European companies to continue to trade with Iran," she told reporters, flanked by Iranian Foreign Minister Mohammad Javad Zarif.

Mr Rouhani said he has no plans to meet Mr Trump while in New York during the marathon of meetings. He has slammed a US offer of talks as "not genuine".

As a precondition for any dialogue, Mr Rouhani said the US president would need to repair the damage done by exiting the nuclear deal. "That bridge must be rebuilt," he told NBC news.

On Wednesday, Mr Trump will for the first time chair a meeting of the Security Council on non-proliferation that will give him a fresh opportunity to make the case for a tougher international stance on Iran.

"The Trump administration's approach toward Iran seems to boil down to: squeeze and let's see what will come," said Robert Malley, president of International Crisis Group.

"Rising tensions between the US and Iran in the absence of diplomatic channels is a recipe for an accidental, perilous clash."