Donald Trump has threatened Iran with “consequences the likes of which few throughout history have ever suffered before” in a Twitter tirade that has stunned diplomatic observers.

The all-caps tweet came after the Iranian President Hassan Rouhani spoke out on America’s anti-Iran policy.

Mr Trump told Mr Rouhani to “never, ever threaten the United States again” and told him to “be cautious!”.

To Iranian President Rouhani: NEVER, EVER THREATEN THE UNITED STATES AGAIN OR YOU WILL SUFFER CONSEQUENCES THE LIKES OF WHICH FEW THROUGHOUT HISTORY HAVE EVER SUFFERED BEFORE. WE ARE NO LONGER A COUNTRY THAT WILL STAND FOR YOUR DEMENTED WORDS OF VIOLENCE & DEATH. BE CAUTIOUS!

— Donald J. Trump (@realDonaldTrump) July 23, 2018

Mr Rouhani had cautioned Mr Trump about pursuing hostile policies against Tehran, saying “America should know … war with Iran is the mother of all wars”.

“Mr Trump, don’t play with the lion’s tail, this would only lead to regret,” Mr Roiuhani said on Sunday, according to local news outlet IRNA.

Tehran faces increased US pressure and looming sanctions after Mr Trump’s decision to withdraw the United States from a 2015 international deal over Iran’s nuclear program.

Iranian President Hassan Rouhani on Monday morning (AEST) cautioned Mr Trump about continuing to pursue hostile policies against Tehran.

Mr Rouhani had left open the possibility of peace between the two countries, at odds since the 1979 Islamic Revolution.

Mr Rouhani warned the US President to not ‘play with the lion’s tail’. Photo: Getty

“You are not in a position to incite the Iranian nation against Iran’s security and interests,” he added, in an apparent reference to reported efforts by Washington to destabilise Iran’s Islamic government.

Mr Trump is still working to regain the support of his Republican party after being widely criticised for his perceived soft approach to Russian President Vladimir Putin at their summit in Helsinki last week.

That meeting culminated a fractious European trip that saw him criticise Germany over its reliance on Russia, demand America’s NATO allies spend more on defence and ridicule British Prime Minister Theresa May’s Brexit deal,

In Washington, US officials familiar with the matter said that the Trump administration had launched an offensive of speeches and online communications meant to foment unrest and help pressure Iran to end its nuclear programme and its support of militant groups.

Clearly something Trump has thought about. https://t.co/GXoGzmuYbo

— Nicholas Kristof (@NickKristof) July 23, 2018

Current and former US officials said the campaign painted Iranian leaders in a harsh light, at times using information that is exaggerated or contradicts other official pronouncements, including comments by previous administrations.

Mr Rouhani scoffed at Mr Trump’s threat to halt Iranian oil exports and said Iran has a dominant position in the Gulf and the Strait of Hormuz, a major oil shipping waterway.

“Anyone who understands the rudiments of politics doesn’t say ‘we will stop Iran’s oil exports’ … we have been the guarantor of the regional waterway’s security throughout history,” Rouhani said, cited by the semi-official ISNA news agency.

Iran’s Supreme Leader Ayatollah Ali Khamenei on Saturday backed Rouhani’s suggestion that Iran may block Gulf oil exports if its own exports are halted.

Mr Trump declared the Iran nuclear deal “rotten to the core” when deciding to pull out in May.

“It is clear to me that we cannot prevent an Iranian nuclear bomb under the decaying and rotten structure of the current agreement,” he said at the time.

The Iran deal is defective at its core. If we do nothing we know exactly what will happen.”

US Secretary of State Mike Pompeo has also launched a rhetorical assault on Iran’s leaders, comparing them to a “mafia” and promising unspecified backing for Iranians unhappy with their government.

Pompeo, in a California speech to a largely Iranian-American audience on Sunday, dismissed Rouhani and Foreign Minister Javad Zarif, who negotiated a nuclear deal with the United States and five other countries, as “merely polished front men for the ayatollahs’ international con artistry.”

Iran “is run by something that resembles the mafia more than a government,” Pompeo said, citing what he called Iranian leaders’ vast wealth and corruption.

Pompeo’s speech was the latest step in a communications offensive launched by the Trump administration that is meant to foment unrest in Iran and help pressure its government to end its nuclear program and support of militant groups, US officials familiar with the matter said.

-with AAP