Washington DC - A portrait of former top US military officer Mark Milley – a foe of Donald Trump – has been taken down at the Pentagon, a source in the building said following the president's inauguration on Monday.

Retired chairmen of the Joint Chiefs of Staff are traditionally honored with portraits that hang in a corridor near the Pentagon's River Entrance, and Milley's had only recently been unveiled at a ceremony earlier this month.

But the source said the retired general's portrait was removed, referring questions about why to the White House.

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

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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 US 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 soldiers. "And we don't take an oath to a wannabe dictator."