HARTFORD, Conn. - Under the watchful eye of law enforcement in 40 states, Craigslist pledged Thursday to crack down on ads for prostitution on its Web sites.

As part of Craigslist's agreement with attorneys general around the country, anyone who posts an "erotic services" ad will be required to provide a working phone number and pay a fee with a valid credit card. The Web site will provide that information to law enforcement if subpoenaed.

Jim Buckmaster, Craigslist's CEO, said the deal will allow legitimate escort services to continue advertising, while providing a strong disincentive to companies that are conducting illegal business.

"We don't view it as a penalty, we view it as raising the accountability," he said. "A legitimate business should have no problem with that. They should have no problem providing a phone number or credit card credentials."

Craigslist filed lawsuits this week against 14 software and Internet companies that help people who post erotic service ads to circumvent the Web site's defenses against inappropriate content and illegal activity.

Craigslist, which posts 30 million ads every month for everything from apartment rentals to jobs in hundreds of cities, will also begin using new search technology in an effort to help authorities find missing children and victims of human trafficking.

Police across the country have been arresting people for using Web sites like Craigslist to advertise the sexual services of women and children.

Connecticut Attorney General Richard Blumenthal, who brokered the agreement, said his office contacted Craigslist after receiving several complaints from users late in 2007 about photographs depicting nudity on the site. He said Craigslist cooperated fully and there was never a need to threaten legal action against the company.

"The dark side of the Internet must be stopped from eclipsing its immense potential for good," Blumenthal said.