On Thursday, a sheriff in Illinois, Tom Dart, Sheriff of Cook County in Chicago, filed a lawsuit against popular online advertising portal Craigslist accusing its owners of knowingly promoting prostitution in the US.

According to legal papers filed in U.S. District Court, the internet-classified website, which runs all manner of ads specific to hundreds of major destinations around the world, is "the single largest source of prostitution in the nation [U.S.]," and is well aware that a certain portion of its user base is openly offering acts of sex in return for money.

"Craigslist is not limited to innocuous postings selling sofas and advertising apartments," the lawsuit states. "Craigslist has an erotic services section where people openly promise sex for money."

Dart said he believes the website makes prostitution easy and wants the owners to close the Erotic Services section of the classifieds. He also noted that a recent FBI study found that Craigslist had more than 2,800 listings for child prostitution.

"Missing children, runaways, abused women and women trafficked in from foreign countries are routinely forced to have sex with strangers because they’re being pimped on Craigslist," exclaimed Dart at a news conference.

"How is that different from somebody who’s aggressively and actively working with a pimp to try to get the word out about the women working for him?" he added.

The lawsuit comes just four months after Craigslist announced plans to implement new policies and technical features designed to purge illegal activity from the site.

As of late last year, Craigslist began charging fees and requiring ID when placing ads in the specified section but there have been many prostitutes who worked around the rules by placing ads in non-paying areas and ordinary sections while disguising their services.

Denying that Craiglist promotes prostitution, a spokeswoman for the website insisted criminal acts are found to be very rare on Craigslist compared to the number of legal activity. Craigslist has stated that the fees and the phone system have both reduced the number of ads in the Erotic Services by 80 percent. They are doing everything they can to prevent this illegal activity from being on the site. This criminal activity is unacceptable to Craigslist and they are doing what they can to prevent it.

"Misuse of Craigslist to facilitate criminal activity is unacceptable, and we continue to work diligently to prevent it," Susan MacTavish Best told InternetNews.com. "Misuse of the site is exceptionally rare compared to how much the site is used for legal purposes. Regardless, any misuse of the site is not tolerated on Craigslist."

The site also pledged to deploy new filtering technology to patrol the site and scrub it of illicit ads. Craigslist is developing new digital tags that will enable parental-screening software to block access to the Erotic Services section.