As if porn sites and pedophiles in chat rooms weren’t frustrating enough for parents whose children use the Internet, now online postings of amateur video featuring skin and violence are raising concerns.

The explosion in online video-sharing sites, where clips of any nature can be easily uploaded for the world to see, has become the latest challenge for parents trying to protect their children and for Web sites coping with obscene submittals.

Carol Kiesman, a mother and teacher in Houlton, Maine, enrolled her 14-year-old daughter in a cyberspace club called “Zoey’s Room” so the teen could chat away online with other girls in a gated community where all participants are screened.

Imagine then how Kiesman cringed when she saw her daughter, 10-year-old son and fourth-grade pupils recently encounter homemade videos online that included nudity and animal cruelty.

“I don’t like that innocent kids can click on stuff like that,” Kiesman said. “What you view as entertainment as an adult shouldn’t be entertainment for 13-year-olds.”

Popular Web sites such as MySpace, YouTube, Yahoo, Google and soon also Microsoft Corp.’s MSN are featuring user-generated videos that quickly have become a phenomenal form of entertainment. YouTube, the leading video site that helped catapult the genre with its public launch in December, attracted more than 20 million visitors in May. The company says it averages 50,000 new video uploads a day.

The infectiousness of the video-sharing sites – users can quickly e-mail friends and family to alert them to favorite videos – has created feverish sensations: The uncanny star of “The Evolution of Dance,” a comedic performance of different dance styles, has amassed more than 25 million page views in two months to become the all-time most-viewed video on YouTube, and the explosive backyard science experiment of mixing Mentos candies with Diet Coke has snowballed into hundreds of copycats, remixes and spinoffs.

Within minutes, an auteur’s work could be viewed by thousands. At some Web sites, videos garnering the most page views are automatically pushed to a highlighted list or “most popular” section.

But alongside the cute animal tricks, comic sports bloopers and corny lip-synching sessions are extremely weird antics and crude clips of bondage or masturbation. There’s a plethora of videos of people vying for some attention and young women flaunting their bodies.

Some viewers, including Ellen Harris of Palo Alto, Calif., consider the racier posts an outgrowth of today’s culture.

“We certainly shake our heads when we see certain stuff, but there’s stuff like that on prime-time TV as well,” said the mother of three teenagers.

Harris thinks the homemade video explosion is an exciting new form of creativity; her family has gathered to watch some hilarious online clips together. The risque byproducts have simply become another source for family discussions – alongside television and movies – on matters such as sex, violence or exploitation.

Still, for now, she has asked her youngest, a 13-year-old daughter, to stay away from MySpace, the leading social networking Web site, which added video-sharing features this year.

To raise parents’ awareness that explicit or inappropriate videos could be accessible to children through popular Web video sites, the New York State Consumer Protection Board last month issued a consumer alert and pushed Google Inc. to do more to protect children.

“Parents have a hard enough time policing the Internet without Google Video making it easier to see and to save these types of videos,” said Teresa Santiago, the board’s chair.

While catering to a mass audience whose entertainment tastes run the gamut, the online video Web sites are aware of the challenges they face in welcoming uncensored clips. They strive to be an open stage for budding musicians, comedians and filmmakers, but they also don’t want to drive away offended viewers or advertisers.

“We are concerned about this issue and are aware that it affects most services that make video available on the Internet,” Google stated in response to the New York consumer board alert.

One dilemma is that while some videos could be considered offensive or inappropriate for underage viewers, they don’t necessarily amount to pornographic or obscene material, which is denounced on YouTube, MySpace, Yahoo and Google.

The Web sites require that those uploading a video sign off on an agreement acknowledging the prohibition of obscene submittals, such as pornography or nudity. But users who click to agree to those terms can ignore it and post anyway, slipping the clips online for a while before they get pulled.

Those top Web sites all rely on viewers to alert them to objectionable clips – a form of community policing that has been used for years by other Internet stalwarts such as auctioneer eBay Inc. and classifieds ads provider Craigslist.

YouTube spokeswoman Julie Supan said “the really objectionable material gets flagged very quickly” – and is pulled from the site usually within 15 minutes. Supan did not disclose specific figures but said “a small percentage” of daily uploads are removed, including those marked for copyright violations.

But not all flagged content gets pulled if the site’s editorial team doesn’t think it violates the user agreement.

Like MySpace, YouTube sometimes keeps the flagged material online but makes the clip accessible only to its registered users who are 18 and older. People who say they’re younger than 13 are barred from registering, though parents and industry observers all know youths could easily work around the age restrictions by logging a false birth year.

“We’re all battling the same thing, keeping this stuff off our site,” Supan said. “But the reality is there’s a handful of people who try to take advantage of the system. And we are trying to put more controls in place.”

Yahoo Inc., which last month added video-upload features to its already vast index of videos culled from throughout the Internet, lets parents turn on a “safe search” mechanism to restrict their children from viewing any content that has been flagged as adult.

And while Yahoo, like its video-sharing rivals, doesn’t prescreen every uploaded video, any clips that get onto its featured pages first must pass the muster of the company’s human editors, said Jason Zajac, Yahoo’s general manager of social media.

Still, Zajac acknowledges the system isn’t perfect. Yahoo is looking into advanced image-recognition technologies that could look for something such as a certain percent of skin tone in an image.

Google Video said it has added more screening methods for videos that appear on its “Top 100” and popular sections. It’s also considering a “safe search” feature similar to Yahoo’s among other improvements.

But even additional human screeners wouldn’t be an end-all answer. “It’s really subjective,” YouTube’s Supan said. “What might offend some might not offend others.”

Consider this recent example: In a four-minute video that was said to be meant as a spoof, a young Marine sings about how he fell in love with an Iraqi woman but then gunned down her family after they confronted him with automatic weapons. The video of his performance, posted anonymously on YouTube, sparked an outcry and was removed.

The Marine has since been cleared by a military review. Now the song is slated to be aired on a satellite radio show and released for commercial downloads.