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.

Craigslist has also agreed to sue 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 ads for everything from apartment rentals to jobs in dozens 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.

“The dark side of the Internet must be stopped from eclipsing its immense potential for good,” Connecticut Attorney General Richard Blumenthal said in a statement Thursday.

The agreement was joined by the National Center for Missing and Exploited Children.

“The criminals engaged in the sexual trafficking of children no longer parade them on the streets of America’s cities,” said NCMEC President and Chief Executive Ernie Allen. “Today, they market them via the Internet.”

States that signed the agreement include Arizona, Arkansas, Colorado, Connecticut, Delaware, Georgia, Hawaii, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Mississippi, Montana, Nebraska, Nevada, New Hampshire, New Mexico, New Jersey, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Utah, Vermont, Virginia, Washington, West Virginia, Wisconsin and Wyoming. Washington, D.C., the U.S. Virgin Islands and Guam also joined.