The US President Donald Trump has vowed a response that will be "1,000 times greater in magnitude," to any attack by Iran.
This is after reports that Iran was plotting to kill the US ambassador to South Africa, Lana Marks.
"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.
Iran's Foreign Ministry spokesman Saeed Khatibzadeh has dismissed the report on the planned assassination that was first reported by Politico.
It reported that Iran was planning to assassinate the US ambassador, a long-time friend of President Trump, in retaliation for the assassination of General Qasem Soleimani in January.
Gen Soleimani was killed in a US drone strike on an airport in Baghdad in January.