Tehran will defend itself on the world stage on Wednesday with President Donald Trump threatening to trigger a new international crisis by pulling out of the Iran nuclear accord.

Iran's President Hassan Rouhani is to address the United Nations General Assembly, where worried world leaders are fearful of a new stand-off even as they grapple with the North Korean menace.

After the speech, the parties to the 2015 nuclear deal will meet, providing a venue for a tense first encounter between Iran's Foreign Minister Mohammad Javad Zarif and US Secretary of State Rex Tillerson.

The stakes are high. Trump is due to report to the US Congress by October 15 on whether he believes Iran is upholding its side of the accord, under which it accepted limits on its nuclear program.

If Congress decides to reimpose economic sanctions -- in the teeth of opposition from fellow deal signatories Britain, China, France, Germany and Russia -- the deal would likely collapse.

US allies in Europe, along with many of Trump's critics in Washington, fear this could trigger a new Middle East arms race even as the world faces an aggressive nuclear-armed Pyongyang.

And if America is blamed for tanking the deal, Trump's ability to lure Kim Jong-Un to the negotiating table may be fatally wounded. 

Nevertheless, the tough-talking US leader did not hold back in his own General Assembly address on Tuesday, sending his clearest signal yet that he intends to torpedo the agreement.

"The Iran deal was one of the worst and most one-sided transactions the United States has ever entered into," he declared, reprising a main theme of his campaign for office.

"Frankly, that deal is an embarrassment to the United States, and I don't think you've heard the last of it -- believe me.

"It is time for the entire world to join us in demanding that Iran's government end its pursuit of death and destruction."

Several of Trump's advisors, including Tillerson and UN ambassador Nikki Haley, have argued that Iran's ballistic missile tests and support for militant groups breaches the "spirit" of the deal.

Now, they want Europe to help them reopen negotiations on a tougher deal which would not allow Iran to resume uranium enrichment in 2025 and would halt missile tests.

"We do need the support, I think, of our allies, our European allies and others, to make the case as well to Iran that this deal really needs to be revisited," Tillerson told Fox News.

The UN nuclear watchdog IAEA says its inspectors have found Iran in technical compliance with the restrictions imposed on its nuclear program, but it is for the deal signatories to decide if the accord is broken.

Representatives of Iran and the powers who signed the Joint Comprehensive Plan of Action (JCPOA) are to meet on Wednesday evening in New York for what may be a stormy session.

It will be the first joint meeting of foreign ministers from the seven signatory countries since the Trump administration took office in January.

- Sunset clauses -

Aside from the US, the other signatories have signaled support for keeping the deal, even if European capitals share Washington's concerns about Iran's subversive activities in the Middle East.

France's President Emmanuel Macron has suggested that more work could be done on measures to restrict Iran's enrichment abilities after the terms of the deal begin to "sunset" in 2025.

But, in his UN address Tuesday, Macron insisted on the importance of the existing deal remaining in place.

"Let's be stricter, but let's not unravel agreements that have brought security," he said, dubbing the accord a "solid, robust agreement that verifies Iran will not build a nuclear weapon."

Tillerson said the "sunset provision" was "the most glaring flaw" and drew a parallel with North Korea, where an agreement on dismantling its nuclear program collapsed in 2002.

"It's not a stiff enough agreement. It doesn't slow their program enough," he said. "We can almost start the countdown clock as to when they will resume their nuclear weapons capability."

For his part, Rouhani warned the United States would only harm its own credibility if it sinks the deal.

"After such a possible scenario, which country would be willing to sit across a table from the United States of America and talk about international issues?" he warned.

"The greatest capital that any country has is trust and credibility."

The renewed doubts over the Iran deal come as Washington and North Korea are also exchanging blood-curdling threats -- as Pyongyang builds a missile arsenal.

The United States has persuaded the UN Security Council to tighten sanctions, hoping to force Kim to come to the table to negotiate the denuclearization of the Korean peninsula.

But if a collapse in the Iran deal damages the credibility of any US counteroffer to North Korea, might the diplomatic route fail and leave the threat of devastating conflict?

- Question of trust -

Iran policy hawks say not, arguing that Trump's uncompromising stance will "signal credibility."

"I think a strong line against Tehran could only help their hand in the case of any prospective with Pyongyang," argued analyst Behnam Ben Taleblu of the Foundation for Defense of Democracies. 

Other experts disagreed. Former State Department policy planner Stewart Patrick, now a fellow at the Council for Foreign Relations, was dismayed by Trump's hardline speech.

"I do think that the North Koreans are watching carefully to see how Iran is treated, and to see what could be expected were they ever in the position to give up their nuclear weapons," he said.

Aside from Rouhani, the highlights of Wednesday's second day of leader speeches at the UN assembly will include Japanese Prime Minister Shinzo Abe and Britain's Theresa May.

Myanmar's second vice president, Henry van Thio, is also due at the podium, at a time when his government is accused of driving more than 420,000 minority Rohingya from their homes.