Schoolgirls as young as 12 are being persuaded to send indecent images of themselves over the internet, according to Sussex Police.
Police officers have been contacted by parents concerned that children using the social messaging system Kik have been approached by people posing as older teenagers.
The children have then been encouraged to take indecent images of themselves and send them to the people they have befriended. They have also been asked to meet those who have contacted them.
Officers are investigating the incidents as potential cases of child grooming. The incidents have taken place in West Sussex, but police have put out a county-wide alert.
They are urging parents to check who their children are speaking to online.
A police spokesman said: “The young people can download Kik for free to their iPhone or Android phone and make contact with people around the world.
“Girls as young as 12 have been persuaded to send indecent images of themselves after being befriended by people claiming to be older teenagers who have also encouraged the children to meet them in person.
“It is unclear how old those who have been approaching the children really are or whether they have asked for the images for their own entertainment or to pass among others.
“We are concerned they may pose a risk to them.
“I would urge parents to speak to their children to warn them about the dangers of making friends online, sending indecent images of themselves and in particular agreeing to meet people who they have only met online.
“Anyone who has any information about adults contacting children online inappropriately should call 101 or the independent charity Crimestoppers anonymously on 0800 555111.”