Iran said on Tuesday that the United States would be siding with the militant Islamic State (IS) group if it designated the Revolutionary Guards as a terrorist organisation.

“The Guards are the defender of the nation,” government spokesman Mohammad Bagher Nobakht told reporters. “If the US wants to put the Guards on the terrorist list, it puts itself in the camp of terrorists."

“Any country that wants to have such a position about the Guards will share this view with the Daesh terrorists,” he said, referring to IS by an Arabic name.

Reports in the Financial Times and other media have suggested US President Donald Trump may call for the Guards to be designated a terrorist group as part of a tougher strategy against Iran to be announced in the coming days.

Analysts say such a move would have a very limited impact on the Guards, who are already subject to a wide range of sanctions.

But Iran's foreign ministry spokesman Bahram Ghasemi threatened a “firm, decisive and crushing” response if it went ahead.

The threat also appears to have unified Iran's establishment, which have often been bitterly divided over the current government's efforts to improve ties with the West.

On Monday night, there was the rare sight of Iran's urbane, globe-trotting foreign minister, Mohammad Javad Zarif, alongside Guards commander Major General Mohammad Ali Jafari appearing together for reporters on the sidelines of a conference in Tehran.

“Diplomatic language is different to the language of defence forces but the content and the objective is the same,” Jafari said.

“Americans would isolate themselves” if they designated the Guards as a terrorist group, Zarif chimed in.

“If American officials make such a strategic mistake, the Islamic republic of Iran will definitely take a reciprocal action. Of course some measures have been planned which we will announce in due time,” he added.

Trump must decide by October 15 whether to recertify Iran's compliance with the nuclear deal it signed with world powers in 2015.

It is widely expected that Trump will not recertify Iran, saying it is no longer in the US national interest, giving Congress 60 days to decide whether to reimpose sanctions.

All the other signatories to the agreement ─ Britain, France, Germany, China and Russia ─ have urged the US to stick with the deal, saying Iran has so far stuck to its commitments to restrict its nuclear programme.

“America's behaviour has frustrated the world,” said Iran's army spokesman Masoud Jazayeri, according to the semi-official ISNA agency.

“It's time America be taught new lessons. Apparently, the Trump government does not understand anything but insults and needs shocks to realise the new concept of power in today's world. “