Donald Trump has threatened Iran, saying an attack on "anything American" will be met with "great and overwhelming force".
The US president also said overwhelming would mean "obliteration" in some areas.
He was speaking after Iran responded to fresh US sanctions against them.
The Trump administration cited "aggressive behaviours" by Iran - including the shooting down of a US surveillance drone - for the new sanctions.
The sanctions target Iranian Supreme Leader Khamenei and his close allies, aiming to block their access to key financial resources and support.
Iran's President Hassan Rouhani today claimed the White House is "mentally retarded", Al Jazeera reports.
He called the new sanctions a "outrageous and idiotic".
Meanwhile, foreign ministry spokesman Abbas Mousavi dismissed the sanctions as 'useless'.
He suggested it means the 'permanent closure' of diplomatic efforts between the two countries.
In remarks quoted by Iran's Mehr news agency, Mr Mousavi claimed: "Trump’s administration is destroying all of the established international mechanisms for maintaining world peace and security."
'Overwhelming will mean obliteration'
In a series of tweets this afternoon, President Trump claimed Iranian leaders only understand "strength and power" - adding that the US is has "by far the most powerful military force in the world".
Tensions are running high between the two countries after the US accused Iran of attacking oil tankers earlier this month.
Iran has denied any involvement in the attacks in the Gulf of Oman.
Following last week's drone downing, President Trump acknowledged the US was "cocked & loaded" to retaliate with military strikes.
However, he claimed he called off the planned action with only minutes to go after hearing that around 150 people would die if the strikes were launched.
All of the recent events also come in the wake of Donald Trump's decision to pull out of the Iran nuclear deal last year.
