US President Donald Trump has said he's given his advisers instructions to obliterate Iran if it assassinates him.

"If they did that they would be obliterated," Trump said on Tuesday in an exchange with reporters while signing an executive order calling for the US government to impose maximum pressure on Tehran.

"I’ve left instructions if they do it, they get obliterated, there won't be anything left."

If Trump were assassinated, Vice President JD Vance would become president and would not necessarily be bound by any instructions left by his predecessor.

Federal authorities have been tracking "Iranian threats" against Trump and other administration officials for years.

Trump ordered the 2020 killing of Qassem Soleimani, who led the Iranian Islamic Revolutionary Guard Corps' Quds Force.

A threat on Trump's life from Iran prompted additional security in the days before a July campaign rally in Pennsylvania where Trump was shot in the ear, according to US officials.

But officials at the time said they did not believe Iran was connected to that assassination attempt.

The Justice Department claimed in November that an Iranian plot to kill Trump before the presidential election had been thwarted.

The department alleged Iranian officials had instructed Farhad Shakeri, 51, in September to focus on surveilling and ultimately assassinating Trump. Shakeri is still at large in Iran.

Iranian officials, at the time, dismissed the allegation, calling the report a plot by Israel-linked circles to make Iran-US relations more complicated.

'Maximum pressure' on Iran

Meanwhile, Trump signed an executive order to reimpose the "maximum pressure" policy on Iran, aiming to curb its oil exports and restrict its influence in the Middle East.

"It's very tough on Iran," he told reporters.

He said he is "torn" on signing the order but added, "Hopefully we are not going to have to use it very much."

The order directs the Treasury Department to impose "maximum economic pressure" on Iran through sanctions designed to cripple the country’s oil exports.

The signing come ahead of his meeting with Israeli Prime Minister Benjamin Netanyahu.

"So I'm signing this, and I'm unhappy to do it, but I really have not so much choice, because we have to be strong and firm, and I hope that it's not going to have to be used in any great measure at all. It'd be great if we could have a Middle East and maybe a world at total peace," said Trump.

In his remarks, he reiterated that Iran cannot have a nuclear weapon.