Musk, the world's richest man and a close ally of President Trump, was speaking at the Capital One Arena in Washington DC when he made the gesture.

"My heart goes out to you. It is thanks to you that the future of civilisation is assured," he said, after giving the second one-armed salute.

There was immediate backlash on social media.

Claire Aubin, a historian who specializes in Nazism within the United States, said Musk's gesture was a "sieg heil", or Nazi salute.

"My professional opinion is that you're all right, you should believe your eyes," she posted on X, in reference to those who believed the gesture to be an overt reference to Nazis.

Ruth Ben-Ghiat, a history professor at New York University, said: "Historian of fascism here. It was a Nazi salute and a very belligerent one too."