Federal officials have warned that online virtual worlds fail to adequately protect children from explicit sexual and violent content, advising parents to take a more active role in their kids' online lives and calling on site operators to clean up their act.

The Federal Trade Commission (FTC) on Thursday released a report analyzing the content and policies of 27 virtual worlds -- immersive, 3-D environments where people interact with each other in the guise of an avatar.

Of those, 19 were home to sexual or violent content, and some went out of their way to encourage minors to circumvent their age restrictions.

"It is far too easy for children and young teens to access explicit content in some of these virtual worlds," FTC Chairman Jon Leibowitz said in a statement. "The time is ripe for these companies to grow up and implement better practices to protect kids."

The overwhelming majority of the explicit content on child-oriented sites described in the report appears in the form of text posted by users on forums or in chat rooms. On sites that cater to adults, half of the content was text-based, with the rest a mix of images, audio, and videos.

The FTC found at least one instance of explicit content in seven of the 14 kid-friendly communities that are open to children under the age of 13. However, the agency said the objectionable content appeared infrequently, and usually only turned up when the researchers had registered on the sites as adults.

The issue gets complicated with teen- and adult-oriented sites, which rely on screening mechanisms to block access to underage users.

Two communities that set a minimum age of 13 denied registrations when FTC staffers entered an age of 12 or younger, but then allowed them to immediately re-register from the same computer when they entered a different date of birth.

Another virtual world, the explicitly adult-oriented Red Light Center, also gave minors a second chance to access the site, but that was only a recent change. The FTC said that when it began its inquiry, the site didn't require users to enter a date of birth to enter the community.

The commission sent letters of inquiry to six virtual-world operators asking about their age-screening techniques.

The FTC called on the companies that maintain these online communities to take a more active role in policing the sites for illicit content and underage users, and to enlist moderators with specific training in dealing with abuses. The commission also suggested that site operators segment members by age to reduce the risk of children interacting with adults, and to reassess the language filters they use to block explicit content.

Kenneth Corbin is an associate editor at InternetNews.com. Based in Washington, D.C., Kenneth's coverage areas range from government regulation to e-commerce and online media.