Hollywood star Nicole Kidman has broken her ankle while isolating in Nashville with her country music singer husband Keith Urban.

The Australian actress's injury has been revealed by Urban after she was photographed by a fan wearing a moon boot.

Kidman, 52, caused consternation among fans about a "mystery" injury after she walked out wearing the moon boot on her right leg.

The photo of Kidman, wearing skinny jeans, an off-the-shoulder peasant top, one black runner and the grey moon boot was taken at Urban's drive-in concert for frontline healthcare workers in Tennessee.

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It was shared on Instagram and Twitter, where users posted comments including "poor Nicole", "is she in pain?" and "wishing a speedy recovery for Nicole".

In the photo, the Oscar winner appears to be blowing a kiss at the photographer, while Urban is holding her hand and looking concerned.

Urban later revealed in an interview about his drive-in concert that his wife had "broken her ankle", but did not reveal exactly how the accident occurred.

On her own Instagram page, Kidman shared a photograph of herself at the concert wearing the same outfit, but her lower legs were cropped out.

Urban staged his hour-long concert on a flatbed truck with two other musicians at the Stardust Drive-In movie theatre outside Nashville on Friday.

About 200 workers from Tennessee's Vanderbilt University Health centre turned up in more than 100 cars for the event, with Urban quipping it was more like "glorified karaoke" because the set-up included a band member playing prerecorded tracks.

It was thought to be the first major live music show of its kind in the US since coronavirus shutdowns cancelled hundreds of concerts and tours and closed large venues in March.

"It was a lot of fun," Urban told Reuters after the show.

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"We didn't know what to expect. For some degree, from a performance standpoint, it was a bit of a proof-of-concept show as well as to figure out what works, what doesn't."

Urban plans to stage more drive-in concerts around America.