The retired general, who reportedly once labelled Trump a "fascist" to a journalist, is the latest official-turned-critic to see their security protection pulled by the Trump administration since the president began his second term last week.

Hegseth informed Milley "that he is revoking the authorization for his security detail and suspending his security clearance as well," Pentagon spokesman John Ullyot said in a statement issued late Tuesday.

Milley was named by Trump during his first administration as the chairman of the Joint Chiefs of Staff, but their relationship deteriorated throughout the Republican's presidency.

Like two other officials who have also had their security details removed, Milley is believed to be under threat from Tehran for overseeing the 2020 US drone strike ordered by Trump that killed powerful Iranian general Qasem Soleimani.

"The secretary has also directed the (Department of Defense inspector general) to conduct an inquiry into the facts and circumstances surrounding Gen. Milley's conduct so that the secretary may determine whether it is appropriate to reopen his military grade review determination," Ullyot said

Milley retired as a four-star general, but that process -- if it goes forward -- could potentially see him demoted in retirement.

In a sign of things to come, Milley's portrait honoring his service as chairman of the Joint Chiefs was taken down at the Pentagon on the day that Trump was sworn in.

The removal of the painting -- the current whereabouts of which are unclear -- came after former president Joe Biden preemptively pardoned Milley and other Trump opponents in one of his last acts in office.

Trump has repeatedly promised "retribution" against his political opponents and threatened some with criminal prosecution, and Biden said he took action to shield Milley and others from "politically motivated prosecutions" under the new administration.

Trump was enraged after Milley told journalist Bob Woodward that the Republican was "fascist to the core" and a "dangerous person."

Milley also revealed he had secretly called his Chinese counterpart after the January 6, 2021 attack on the US Capitol by Trump's supporters to reassure Beijing that the United States remained "stable" and had no intention to attack China.

Trump subsequently wrote on his Truth Social network that "in times gone by, the punishment would have been DEATH!" for Milley.

The general stepped down as chairman in 2023 at a ceremony in which he took a final swipe at Trump.

"We don't take an oath to a king, or queen, or a tyrant or a dictator," Milley said of American troops. "And we don't take an oath to a wannabe dictator."