US President Donald Trump used his UN General Assembly speech on Tuesday (Sep 22) to make the case for his war on Iran, arguing he has prevented Tehran from obtaining a nuclear weapon and warning that he could "annihilate the Islamic Republic" if no deal is reached to end the conflict.

While predicting his administration would strike a peace deal with Iran following the Nov 3 US midterm elections, Trump called on other nations to join America in isolating Iran economically to force it to the negotiating table.

"I have a big decision to make. Will a deal be made with Iran that lets them rebuild and create a far greater country than it ever was before?" Trump said. "Or do I annihilate the Islamic Republic and do it quickly, never giving them a chance to kill and destroy people and countries again?"

He later told reporters at the United Nations that US officials had had a "very good" three-hour meeting on Tuesday with the Iranian delegation.

Since returning to office last year for a second term, Trump has sent waves through the global order, joining Israel to attack Iran, ordering a raid on Venezuela that captured the country's leader, and threatening to seize Greenland.

In a 37-minute speech ahead of a whirlwind diplomatic schedule that included meetings with the leaders of Britain, Ukraine and Japan, Trump laid out his foreign policy vision, arguing that the Iran war had spared the world the threat of a nuclear-armed menace while asserting US dominance in the Western Hemisphere is essential to preventing crises from taking root.

"Our actions in Venezuela are proof that the United States will no longer permit threats to America to gain a foothold anywhere in the Western Hemisphere," Trump said. "While others have spoken of peace, I have made peace."

Outside the UN headquarters, dozens of anti-Iranian-government demonstrators gathered, a few of them holding up signs calling for democracy and decrying dictatorship.

Trump brushed aside suggestions that the midterm elections, in which his Republican Party risks losing control of Congress, could constrain his handling of the war. Some analysts have argued that a Democratic takeover of the legislature would weaken his leverage in the conflict.

"What they don't realize is that I'm not running," Trump said, referring to the leadership in Tehran. “I did that already, and won in a landslide. We won in a giant landslide, and I'll be here for 2-1/2 years.”

Lincoln Mitchell, an adjunct associate research scholar at Columbia University’s Arnold A. Saltzman Institute of War and Peace Studies, said Trump’s prediction that a deal with Iran would come after the midterms reflected the difficulty of reaching an agreement in the near term.

“If he could get a deal tomorrow, he would,” Mitchell told CNA’s Asia First, saying that Iran is under “no time pressure to do this”.

Mitchell said securing an agreement before the elections would be advantageous for Trump, particularly if an end to the conflict helped bring down energy prices.

“The only kind of piece of ammunition, so to speak, that Trump has left is to threaten massive, horrible destruction of Iran,” he said.