LOS ANGELES (AFP) - Actresses Emma Watson and Amanda Seyfried launched legal action on Wednesday (March 15) after private pictures were stolen and posted online, while Mischa Barton spoke of her anguish over a leaked sex tape.

Although the photographs of Watson merely show her trying on outfits, the images of Seyfried and Barton show them in a series of compromising and sexually explicit poses.

Watson's publicist said her stolen pictures were taken during a fitting she had with a stylist "a couple of years ago", without offering further details about the circumstances of the shoot or how the images had ended up in the wrong hands.

"They are not nude photographs. Lawyers have been instructed and we are not commenting further," he said.

Media reports said the pictures had been shared on the so-called "dark web" - an encrypted part of the internet not easily accessible by users lacking specialist knowledge.

Later on Wednesday, Seyfried's legal team said it was pursuing a website that had posted her nude photos, the celebrity gossip portal TMZ reported, including images of "intimate moments" with her ex-boyfriend.

TMZ posted what it described as a threatening letter from the actress's lawyer to Celeb Jihad, demanding that it immediately take down the images.

It was not immediately clear if the pictures, which remained available on the site in the early afternoon Los Angeles time, were obtained in an attack linked to the Watson theft.

Meanwhile, Barton told a news conference on the outskirts of Los Angeles that she was taking action to stop the potential sale of a sex tape made against her will in a so-called "revenge porn" case.

She said she had obtained a restraining order, and that cease-and-desist letters had been sent to those believed to be trying to buy or sell the images.

"I just want to say that I have been put through an incredibly hard and trying time," said the actress best known for her starring role in the Fox series The OC. Reading from a prepared statement, she said her worst fears were realised when she learned that someone she had loved and trusted had been filming her "most intimate and private moments" with hidden cameras.

"I came forward to fight this, not only for myself but for all the women out there," she said. "I want to protect them from the pain and humiliation that I have had to go through."

Barton's lawyer, Lisa Bloom, said someone the actress had once dated was offering the tape for US$500,000 (S$701,811), adding that buyers could be prosecuted.

The images were recorded during the last year, she said, refusing to identify the culprit, citing an ongoing police investigation.

The theft comes amid a spate of cases in which female celebrities had private and potentially embarrassing images leaked online, sometimes by those intending to blackmail their victims.

Watson, who stars in Disney's Beauty And The Beast, due to hit United States theatres on Friday, was threatened on the controversial 4Chan message board in 2014 with a leak of nude images after she had delivered a speech about gender equality. That threat turned out to be a hoax.

Last month, a 29-year-old Chicago man was jailed for nine months for hacking into the electronic accounts of celebrities - including Jennifer Lawrence - on Apple's iCloud service, and stealing private information including nude videos and photos.

Dozens of stars' photos were posted online as part of the 2014 phishing exercise, including shots of Rihanna, Kate Upton and Kirsten Dunst. Apple admitted that there was a "targeted attack", but denied that its cloud storage system had been breached.