Microsoft, AOL and the Safe Internet Alliance are joining forces to urge parents to discuss online safety with their children.

Kim Sanchez, senior audience marketing manager in the Trustworthy Computing group at Microsoft, Linda Criddle, president of Safe Internet Alliance and Holly Hawkins, AOL's director of consumer advocacy and privacy are warning parents that with the summer holidays approaching, kids will be spending even more time online.

Parents should know what their children are doing online, what social-networking sites they frequent, and whether they use their mobile phones to access the internet, said the three safety experts.

Sanchez also said parents should communicate with teens and remind them of the dangers of sharing personal information, posting sexually explicit photos and meeting people offline.

"Kids need to think about what they're putting out there for everybody to view before they do it," Sanchez said. "They should think before they post a blog post, or put comments on their page, or photos. They need to consider this could be seen by anybody on the internet, and it may be permanent."

A recent Harris Interactive poll found that one in five 13 to 18 year olds have sent nude pictures of themselves through their mobile phones or email accounts. The poll also revealed that 11 percent of those teens who've sent nude photos have sent them to strangers.

The number of teens saying they've sent sexually explicit photos to strangers was "shocking," Criddle said.

Mobile phones offer teens an opportunity to send pictures spontaneously, and some teens don't think before sending out embarrassing photos, said Hawkins. "It leaves very little time, if any, for having a second thought," she said.

Generally, offering guidance is more effective than trying to control children's online behavior, Sanchez said.

"Parents need to negotiate clear guidelines for their kids' internet use," she said. "There's really no technological substitute for parental involvement, supervision and guidance."

Parents can use parental controls to monitor or block much online content, Hawkins said. "It's important to realise that, just as in the offline world, parenting online never stops, its a never-ending cycle," she said.

Parents should get online and use the technologies their kids are using, Criddle added. "So many parents seem to be just sort of afraid of jumping in," she said.

"They think that somehow they're going to break something. They don't have that same willingness to just jump into a product and try it out that their kids do."