Washington DC - Donald Trump said Wednesday that Iran should be blown "to smithereens" if the Islamic Republic is involved in the harming of a US White House candidate or ex-president.

The provocative remarks come after American intelligence warned of threats from Tehran against the Republican's life after two apparent assassination bids in recent months.

"As you know, there have been two assassination attempts on my life that we know of, and they may or may not involve – but possibly do – Iran," Trump said at a campaign event in North Carolina.

"If I were the president, I would inform the threatening country, in this case, Iran, that if you do anything to harm this person, we are going to blow your largest cities and the country itself to smithereens," he added.

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Trump went on to say he and the US have been "threatened very directly by Iran" and that a firm message needed to reach Tehran that there would be the most severe consequences should it be involved in plots to kill or hurt an American president or candidate.

"The best way to do it is through the office of the president, that [if] you do any attacks on former presidents or candidates for president, your country gets blown to smithereens, as we say."

Trump also said it was "strange" that Iranian President Masoud Pezeshkian was in New York this week and accorded substantial protection as he attended the United Nations General Assembly even as news of the threats emerged.

"We have large security forces guarding him, and yet they're threatening our former president and the leading candidate to become the next president of the United States," Trump said.

The remarks come as world leaders scramble to try to avert hostilities between Iran-backed Hezbollah and US-backed Israel escalating into a wider regional war.

Iran has rejected accusations it is trying to kill Trump this summer, shortly after a gunman opened fire at a rally in Pennsylvania on July 13, killing one person and wounding the presidential candidate.

Days later, Trump posted on social media that if Iran did kill him, "I hope that America obliterates Iran, wipes it off the face of the Earth."

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On Wednesday, the 78-year-old Trump suggested the would-be assassin in Pennsylvania had used "potentially foreign-based apps" and that the alleged gunman in Florida had multiple mobile phones that Trump said US authorities have been unable to open.

"They must get Apple to open these foreign apps [and] open the six phones from the second lunatic," Trump said. "Because we have a lot at stake."