PHILADELPHIA (Reuters) - The American Civil Liberties Union sued a Pennsylvania prosecutor on Wednesday over his threats to charge three teenage girls with child pornography for allowing themselves to be photographed partly clothed with cellphone cameras.

The case involves the growing practice among teens of “sexting,” a play on the term texting, in which nude or semi-nude photos are sent on cell phones or posted on the Internet.

The ACLU of Pennsylvania, a chapter of the civil rights group, said Wyoming County District Attorney George Skumanick accused the girls of being accomplices to the production of child pornography because they allowed themselves to be photographed. Other unidentified people distributed the pictures.

Pictures showing two of the girls wearing white bras, and another standing topless with a towel wrapped around her waist were discovered by school officials in October 2008, the ACLU said. The pictures did not show any sexual activity.

The ACLU said Skumanick should not have threatened to file felony charges against the girls unless they agreed to be placed on probation and participate in a counseling program. The ages of the girls were not given.

“Kids should be taught that sharing digitized images of themselves in embarrassing or compromised positions can have bad consequences, but prosecutors should not be using heavy artillery ...to teach them that lesson,” said Witold Walczak, ACLU Pennsylvania legal director.

A 2008 national survey found 20 percent of teenagers say they have sent or posted online nude or semi-nude pictures of themselves, and 39 percent say they have sent or posted sexually suggestive messages, according to the National Campaign to Prevent Teen and Unplanned Pregnancy.

Sara Mullen, a spokeswoman for the Pennsylvania ACLU, said the case could set a national guideline for schools, prosecutors and parents over how to deal with the “sexting” phenomenon.

The ACLU lawsuit asks a federal judge to bar the district attorney from filing criminal charges against the girls. The suit claims Skumanick misused his authority by threatening to bring baseless child-pornography charges in order to force parents to enroll their daughters in the proposed probation and counseling program.

Skumanick said he will argue at a federal court hearing in Scranton, Pennsylvania on Thursday that the ACLU has no basis for its suit. He said no charges have been filed because he wanted to give the girls a choice between criminal charges and the other proposed remedies.

“We gave them the option which we didn’t have to do,” he said. “We thought we were trying to be proactive.”