The US is reportedly preparing to designate Iran‘s elite Islamic Revolutionary Guards Corps (IRGC) as a foreign terrorist organisation.

It would be the first time Washington has formally labelled another country’s military a terrorist group.

The decision is expected to be announced by the US State Department perhaps as early as next week, The Wall Street Journal reported. It has been rumoured for years.

Iran warned it might put the US military on its terror list in retaliation, a senior Iranian politician said on Saturday.

“If the Revolutionary Guards are placed on America’s list of terrorist groups, we will put that country’s military on the terror blacklist next to Daesh [Isis],” Heshmatollah Falahatpisheh, head of the Iranian parliament’s national security committee, tweeted.

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US secretary of state Mike Pompeo has advocated for the change in US policy as part of the Trump administration’s tough posture towards Tehran.

The announcement would come ahead of the first anniversary of president Donald Trump‘s decision to pull out of a 2015 nuclear deal with Tehran and to reimpose sanctions that had crippled Iran’s economy.

The US has already blacklisted dozens of entities and people for affiliations with the IRGC, but the organisation as a whole is not.

In 2007, the US Treasury designated the IRGC’s Quds Force, its unit in charge of operations abroad, “for its support of terrorism". It has described it as Iran’s “primary arm for executing its policy of supporting terrorist and insurgent groups”.

Iran previously warned of a “crushing” response should the US go ahead with the designation.

IRGC commander Mohammad Ali Jafari warned in 2017 that if Mr Trump went ahead with the move “then the Revolutionary Guards will consider the American army to be like Islamic State all around the world”.

Iran marks 40 years of the Islamic Republic

Such threats are particularly ominous for US forces in places such as Iraq, where Iran-aligned Shia militia are located in close proximity to US troops.

Set up after the 1979 Islamic Revolution to protect the Shia clerical ruling system, the IRGC is Iran’s most powerful security organisation. It has control over large sectors of the Iranian economy and has a huge influence in its political system.

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The IRGC is in charge of Iran’s ballistic missiles and nuclear programmes. Tehran has warned it has missiles with a range of up to 2,000km (1,242 miles), putting Israel and US military bases in the region within reach.

The IRGC has an estimated 125,000-strong military with army, navy and air units and answers directly to supreme leader Ayatollah Ali Khamenei.