Apple said Tuesday a "targeted attack" led to the release of nude photos of celebrities including Oscar winner Jennifer Lawrence but insisted there was no breach of its cloud storage system.

The admission came as experts and lawyers said the hack was a wake-up call about the dangers posed by technology to people's privacy, whether they are stars or not.

"After more than 40 hours of investigation, we have discovered that certain celebrity accounts were compromised by a very targeted attack on user names, passwords and security questions, a practice that has become all too common on the Internet," Apple said.

"None of the cases we have investigated has resulted from any breach in any of Apple's systems including iCloud or Find my iPhone. We are continuing to work with law enforcement to help identify the criminals involved."

The statement was the first since the release at the weekend of private, nude pictures of dozens of celebrities including "Hunger Games" star Lawrence and top model Kate Upton.

"When we learned of the theft, we were outraged and immediately mobilized Apple's engineers to discover the source," the Apple statement said.

The Federal Bureau of Investigation confirmed it was investigating.

"The FBI is aware of the allegations concerning computer intrusions and the unlawful release of material involving high-profile individuals, and is addressing the matter," the US law enforcement agency said.

"Any further comment would be inappropriate at this time," added Laura Eimiller, FBI spokeswoman in Los Angeles.

The Apple statement suggested that the celebrities had their accounts hacked by using easy-to-guess passwords, or by giving up their personal data to cybercriminals posing as Apple, a technique known as "phishing."

Dozens of stars hacked

Hundreds of nude pictures of the stars were apparently posted, causing shockwaves in Hollywood and in the computer security community.

Among the dozens of celebrities whose pictures were allegedly stolen were singer Avril Lavigne, actress Hayden Panettiere and United States soccer star Hope Solo.

Former Nickelodeon star and singer Victoria Justice said the images claiming to show her nude were anything but the real deal.

Celebrity lawyer Gloria Allred told AFP that public figures needed to understand the risks posed by ever-advancing technology.

"If you wouldn't want to see it on tomorrow's front page don't sent it via unencrypted email or store it in public cloud services, or on your cell phone for that matter," Allred said.

Chris Morales, a security specialist with NSS Labs, told AFP the hack appeared to be going on for some time, accessing multiple accounts -- not just of celebrities but of their friends and associates.

"The pictures weren't just from celebrity accounts," Morales said.

"They might have been from friends or ex-boyfriends or other people. Someone did a lot of digging."

The leak of private celebrity images recalls a similar case from 2011, in which a hacker broke into the email accounts of stars including Scarlett Johansson and Christina Aguilera.

Christopher Chaney, 36, was subsequently ordered to pay $76,000 to victims, after pleading guilty in a deal with prosecutors. He could have been jailed for up to 121 years if he had been convicted on all charges.

Morales added that it is possible that the person or persons who leaked the pictures were not the hackers, but obtained the photos later on.

"The people who put it online were trying to trade photos for bitcoins," he said.

"It was claimed that this was a sampling of a bigger cache."

Celebrities took to Twitter to denounce the latest hack.

"Girls" creator Lena Dunham said: "Remember, when you look at these pictures you are violating these women again and again. It's not okay ...The person who stole these pictures and leaked them is not a hacker: they're a sex offender."

"Thank you iCloud," wrote actress Kirsten Dunst, reportedly one of those involved.