US National Security Advisor John Bolton cited "a number of troubling and escalatory indications and warnings" when he announced the deployment of the USS Abraham Lincoln Carrier Strike Group and a bomber task force on May 5.

Since then, the US Defense Department said the deployments were "in response to indications of heightened Iranian readiness to conduct offensive operations against US forces and our interests."

And then on Monday, Trump put Tehran on notice that any action against US interests would not be tolerated.

"If they do anything, it would be a very bad mistake," the Republican leader said.

While he did not expand on the nature of the threats, several US defense officials said he was referring to arms shipments and personnel movements by pro-Iranian groups, on land and at sea, as well as orders for those groups to attack US interests in the Middle East.

Those targets could include diplomatic installations, as well as consulates or embassies.

The officials, who spoke on condition of anonymity, suggested that the Pentagon was suffering the consequences of Trump's aggressive stance, notably his decision to put Iran's elite Revolutionary Guards and its prized Quds force on the US terror blacklist.

Iran quickly dubbed the United States a "state sponsor of terrorism" and designated Centcom and its forces a "terrorist group."

- Belligerent tone -

From that moment on, Americans became fair game, said one of the US officials, who said he is taking Tehran's threats very seriously.

More than 5,000 US troops are deployed in Iraq, where they are helping in the fight to clear the remaining elements of the Islamic State group, despite a declaration of the end of its "caliphate" stretching into Syria.

While the Revolutionary Guards officially have no presence in Iraq, they are still influential and were instrumental during the heat of the fight against the IS group, with the head of its foreign wing Major General Qassem Suleimani coordinating fighting across various Iraqi battlefields.

The Guards are in Syria, where the US has 2,000 military personnel fighting alongside the Kurdish-led Syrian Democratic Forces (SDF).

For months, this close proximity has worried the US military, which fears for its soldiers on the ground as diplomatic tensions soar.

It's one of the reasons cited by former defense secretary Jim Mattis when he publicly opposed Washington's withdrawal from the multilateral Iran nuclear deal and the resumption of US economic sanctions on Tehran.

And when the White House said earlier this month that Trump was considering placing the Muslim Brotherhood -- a nearly century-old Islamist movement born in Egypt with pockets of support across the Arab world -- on the terror blacklist, many in the administration objected, especially at the Pentagon, The New York Times reported.

On Monday, in stark contrast to Trump's belligerent tone, US defense officials who spoke to AFP tried to tamp down the uproar.

"We have made very clear that we are not interested in a conflict with Iran," one of them said.

The US military deployment in the Gulf is "not an invasion force -- it is a force that is made to send a message."