The FBI is investigating after nude photos of Oscar-winning actress Jennifer Lawrence and other female stars were stolen and posted online.
The intimate images first appeared on an online bulletin board in the US before going viral.
More than 60 photographs of Lawrence were among those reportedly stolen.
A representative for the US actress confirmed the images are genuine.
"This is a flagrant violation of privacy. The authorities have been contacted and will prosecute anyone who posts the stolen photos of Jennifer Lawrence," a statement said.
Initial reports suggested Apple's iCloud service had been compromised, letting hackers access the accounts of celebrities.
Apple said it was "actively investigating" the allegations.
The FBI, meanwhile, confirmed it was "addressing the matter," although offered no further comment.
A list of those who have allegedly been hacked has appeared on the web, Buzzfeed reported.
It contains more than 100 names, including Kate Upton, Cara Delevingne, Aubrey Plaza, Cat Deeley, Kelly Brook, Kim Kardashian, and Rihanna.
The hacker also claims to have a "full set" of photographs of US soccer star Hope Solo.
Actress Mary Elizabeth Winstead, who was on the list, tweeted: "To those of you looking at photos I took with my husband years ago in the privacy of our home, hope you feel great about yourselves."
In another post she said: "Knowing those photos were deleted long ago, I can only imagine the creepy effort that went into this. Feeling for everyone who got hacked."
But a spokesman for the singer Ariana Grande said photographs that claimed to show her were fake.
And the actress Victoria Justice also denied images purporting to show her were real.
She tweeted: "These so called nudes of me are FAKE people."
British actress Emma Watson, who is not thought to have been affected, wrote on Twitter: "Even worse than seeing women's privacy violated on social media is reading the accompanying comments that show such a lack of empathy."
The FBI has investigated previous leaks of nude celebrity images, including those of Scarlett Johansson and Christina Aguilera.