Donald Trump, the US president, warned Tehran it would face severe military retaliation if reports that it is planning to assassinate a US diplomat prove to be true.
"Any attack by Iran, in any form, against the United States will be met with an attack on Iran that will be 1,000 times greater in magnitude!” Mr Trump tweeted on Monday evening.
The threat comes a day after Politico reported that Iran was weighing up an assassination attempt against Lana Marks, the US ambassador to South Africa and long-time friend of President Trump.
The reported plan is said to be in response to the killing of top Iranian commander General Qassim Soleimani by US forces in January this year.
The report was based on documents seen by an anonymous US intelligence official and another who was “familiar with the issue”.
Ms Marks had been made aware of credible threats to her life, they said. The reporting has not been confirmed by any other major US news organisations or the US government.
Serious tensions have been rising between the two countries since the assassination of Gen Soleimani, the leader of Iran’s elite Quds force, in January.
For months Iran has been weighing up different ways to retaliate for the killing of their revered commander who caused devastation across the world’s conflict zones.
War was feared when Iran launched attacks on US forces in Iraq in response, wounding over 100 troops.
The threat de-esclated after Mr Trump walked back on his promise to use disproportionate force in retaliation for the attack against US forces and chose to use economic sanctions as a weapon instead, but tensions have remained high.
Mr Trump said on Tuesday he also wanted to order an assassination of Syrian President Bashar al-Assad after a chemical attack in 2017, but James Mattis, his former Secretary of Defense, "was against it."
"I would have rather taken him out. I had him all set. Mattis didn't want to do it," the president said on Fox & Friends.
"I don't regret that. I could have lived either way with that. I considered him, certainly not a good person. But I had a shot to take him out if I wanted and Mattis was against it. Mattis was against most of that stuff. He'd keep you in military but he didn't know how to win."
He had previously denied any operation to kill Assad, telling reporters it "was never even contemplated."
The latest incident comes as President Trump campaigns for re-election against Joe Biden, who has previously accused him of bringing the US closer to war with Iran without a real strategy.
An intelligence source told Fox News the threat is being taken seriously and Ms Marks is among "several US officials that American intelligence agencies believe Tehran is considering for retaliation for the killing of Soleimani." Iran denies the reports.
According to Politico’s anonymous intelligence source, the US believes a plot against Ms Marks has been on the cards since Spring, but has been made more specific in recent weeks.
Ms Marks, 66, was sworn in as ambassador in October. While she does not have any known ties to Iran, Politico reported that her more than two-decades-long friendship with the US president may have made her a target.