Intimate images of dozens of celebrities, including Academy Award winner and Hunger Games star Jennifer Lawrence, have been leaked online by an anonymous hacker who claims to have stolen materials from more than 100 actors and singers.

A user on the internet bulletin site 4chan began posting the images, which included nude photographs of Lawrence, Sports Illustrated model Kate Upton and other movie and television personalities, shortly after 4 p.m. ET Sunday.

Some actors took to Twitter to deny the images were of them, saying they were fabricated, while others acknowledged the authenticity of the photos but expressed anger at those who posted them.

Lawrence has contacted authorities to investigate who accessed and posted the pictures, a publicist for the actress said.

"This is a flagrant violation of privacy," Lawrence's publicist Liz Mahoney wrote in a statement. She declined to provide further details, including which authorities were contacted.

The FBI said on Monday that it would be "addressing the matter," but did not state what steps the agency would be taking to investigate, reported the Associated Press, citing a statement from spokeswoman Laura Eimiller.

Various media outlets are reporting that private images and video were accessed through a security weakness in the celebrities' Apple iCloud accounts, but Apple has not confirmed that claim. 

The same user also posted a list that included more than 100 names and claimed images and video of people on that list will be uploaded online in the following days.

Celebrity gossip blogger Perez Hilton initially posted some of the photographs on his site but then pulled them down and tweeted an apology, saying he had acted in haste.

Various media are also reporting that numerous Twitter accounts whose owners reposted some of the images in question were suspended.

Avril Lavigne reportedly on list

According to the Guardian newspaper, the list of celebrity names includes Canadian singer Avril Lavigne, reality star Kim Kardashian, singer Rihanna and actress Kirsten Dunst.

FBI spokeswoman Laura Eimiller declined comment on whether the agency was investigating the hacking.

The FBI has investigated previous leaks of nude celebrity images, including leaks involving Scarlett Johansson, Mila Kunis, Christina Aguilera and footage of television sports reporter Erin Andrews in a Tennessee hotel room. Those cases resulted in convictions.

Steph Guthrie, feminist and social media consultant

Hackers who delight in posting private, intimate images of actresses online are 'taking pleasure in sexually humiliating young women,' social media consultant and feminist activist Steph Guthrie says. (CBC)

In 2012, Christopher Chaney of Florida was sentenced to 10 years in U.S. federal prison after he hacked into the personal devices of actress Scarlett Johansson and posted revealing images online.

Some cybersecurity experts speculated that hackers may have obtained a cache of private celebrity images by exploiting weaknesses in an online image-storing platform.

"It is important for celebrities and the general public to remember that images and data no longer just reside on the device that captured it," security researcher Ken Westin wrote in a blog post Monday. "Once images and other data are uploaded to the cloud, it becomes much more difficult to control who has access to it, even if we think it is private."

Toronto-based social media consultant and feminist activist Steph Guthrie said it's not at all uncommon for adults to sext, which could explain why anyone, including celebrities, might have explicit images of themselves on their mobile devices and accounts.

But, she said, it amounts to trampling on people's rights to privacy to hack into those accounts and steal the photos.

"A lot of time, they're doing it as a way of demonstrating status and saying, ' My technical skills and my access are greater than yours. Look what I can achieve'," Guthrie said.

"It's a show of dominance. It's a show of, 'Oh, look what I can have access to.' And it's taking pleasure in sexually humiliating young women."

Oscar Nominations

American actress Jennifer Lawrence, shown in a scene from American Hustle, is among celebrities at the centre of the latest online leak scandal. Lawrence's publicist confirmed that some images of the actress in the nude were stolen. (Francois Duhamel/Associated Press)

With files from The Associated Press