Mandy Moore has slammed "rude" Instagram trolls after they accused the singer/actress of digitally altering a photo online to make her waist appear slimmer.

The star shared a few snaps of herself backstage at The Tonight Show Starring Jimmy Fallon on Tuesday (10Oct17) and expressed her excitement at promoting the second season of her hit TV drama This Is Us.

"Love any time I get to hang with the sweetest @jimmyfallon on @fallontonight," she captioned the pictures, two of which had been taken side-on.

Fans were quick to praise Mandy for her slender figure, but some followers criticized her for the skinny appearance and claimed the photos had been altered using image editing program Photoshop.

"Why are the lines of the wood panels straight until her midsection? That's a very clear indicator of Photoshop," remarked one Instagram user, as another posted, "Seriously, with a bicep that looks like that, Photoshop took over the waist. Unfortunate from you...no one's waist is that thin (sic)".

The backlash prompted Mandy to hit back, responding directly to one hater, "Not nice or true. Do better," adding to another, "This is so disheartening. Nope just the angle. Ugh".

She has since updated the caption for her Tonight Show photos to make it clear she isn't the kind of person to edit images she posts on social media.

Mandy declared: "I am 5'10 and a size 6. I have NEVER photoshopped pictures. That's not what I'm about. If you're going to be rude about people's bodies, go elsewhere."