It is the first such designation by any American administration of an entire foreign government entity, although portions of the Guard, notably its elite Quds Force, have been targeted previously by the US.

Iran’s foreign minister, Mohammad Javad Zarif, seemed to anticipate the designation, saying in a tweet on Sunday aimed at President Donald Trump that Trump “should know better than to be conned into another US disaster”.

The designation comes with sanctions, including freezes on assets the Guard may have in US jurisdictions and a ban on Americans doing business with it or providing material support for its activities.

Although the Guard has broad control and influence over the Iranian economy, such penalties from the US may have limited impact.

The designation, however, could significantly complicate US military and diplomatic work.

Watch: When the US reimposed all sanctions on Iran

The Pentagon and US intelligence agencies have raised concerns about the impact of the designation if the move does not allow contact with foreign officials who may have met or communicated with Guard personnel.

Those concerns have in part dissuaded previous administrations from taking the step, which has been considered for more than a decade.

It was not immediately clear whether the designation would include such carve-outs.

In addition to those complications, American commanders are concerned that the designation may prompt Iran to retaliate against US forces in the region, and those commanders plan to warn US troops remaining in Iraq, Syria and elsewhere of that possibility, according to a US official.

The official was not authorised to discuss the matter publicly and spoke on condition of anonymity.

Aside from Iraq, where some 5,200 American troops are stationed, and Syria, where some 2,000 troops remain, the 5th Fleet, which operates in the Persian Gulf from its base in Bahrain, and the Al Udeid Air Base in Qatar, are potentially at risk.

Despite the risks, Iran hardliners on Capitol Hill and elsewhere have long advocated for the designation.

They say it will send an important message to Iran as well as deal it a further blow after Trump pulled out of the 2015 nuclear deal and reimposed economic sanctions.

Secretary of State Mike Pompeo and national security adviser John Bolton have taken up the call and in recent months spoke stridently about Iran and its “malign activities” in the region.

Pompeo has made clear in public comments that pressure on Tehran will only increase until it changes its behaviour.

Just last week, Pompeo’s special representative for Iran, Brian Hook, accused Iran and its proxies of being responsible for the death of 608 US troops in Iraq between 2003 and 2011.

He cited newly declassified Defence Department information for the claim.

“Secretary Pompeo will continue to use all the tools at our disposal to press the regime to change its destructive policies for the benefit of peace in the region and for the sake of its own people, who are the longest-suffering victims of this regime,” Hook said.

The department currently designates 60 groups, such as al-Qaeda and Islamic State and their various affiliates, Hezbollah and numerous militant Palestinian factions, as “foreign terrorist organisations”. But none of them is a state-run military.

Once a designation is announced by the secretary of state in coordination with the Treasury secretary, Congress has seven days to review it. If there are no objections, it then will take effect.

Additional reporting by Agence France-Presse