American teenagers are increasingly facing prosecution as sex offenders as a result of the rapidly spreading practice of sending explicit photos of themselves by mobile phone — a trend known as "sexting".

In the latest case, three teenage girls in Greensburg, Pennsylvania, who sent nude self portraits, and the three male classmates who received the images, have all been served with child pornography charges.

The girls have been charged with manufacturing and disseminating child pornography while the boys are accused of possessing it.

In Wisconsin, a 17-year-old was charged with child pornography after posting naked pictures of his girlfriend, who is a year younger, on the internet. In Rochester, New York, a boy aged 16 faces seven years in jail for circulating an image of a girlfriend to friends.

"Sexting" is fast becoming a moral and legal headache for school heads and police throughout America. The National Campaign to Prevent Teen and Unplanned Pregnancy last month published a study suggesting one in five teens had sent or posted images of themselves in various stages of undress.

Jim Brown, an official at Glen Este high school in the Ohio town of Cincinnati, told the Cincinnati Enquirer: "If I were to go through the cell phones in this building right now, of 1,500 students I would venture to say that half to two- thirds have indecent photos, either of themselves or somebody else in school."

Prosecutors are facing increasing dilemmas because case law has not kept up with the impact of digital media on teenage behaviour. Young adults can face lengthy sentences resulting from relationships with younger teenagers, with penalties varying state by state.

Federal law also requires hefty punishment for teenaged relationships that span the legal start of adulthood at 17. An 18-year-old in their last year of high school who dates a 14-year-old in the first year faces up to 30 years in jail for a first offence.