An American teenager accused of exploiting two young Australian girls online by threatening to release inappropriate photographs has been charged with sexual extortion.

Key points: Kai Ranaglia-Nelson, 18, was charged with two counts of sexual extortion

Kai Ranaglia-Nelson, 18, was charged with two counts of sexual extortion He allegedly received inappropriate photographs of two underage Australians and threatened to release them

He allegedly received inappropriate photographs of two underage Australians and threatened to release them US authorities say more victims may come forward

The girls, aged 13 and 14, came forward to authorities in New South Wales last year.

The inappropriate photographs were shared between Kai Ranaglia-Nelson, 18, and the girls before he allegedly threatened to expose them if they did not continue to send more material.

Australian detectives contacted their counterparts in the Pinal County Sheriff's Office, Arizona, who then tracked the suspect to the Coolidge area.

In a statement, Sheriff Mark Lamb said Mr Ranaglia-Nelson admitted his involvement when interviewed by detectives.

"This is an important reminder to monitor your children's social media account and know who they are talking to," he said.

"It is so important to talk to your children tonight about the dangers of sharing inappropriate photos with anyone.

"We see cases all the time where those victims get extorted for the photos they send."

The crime occurred not just locally, but internationally, he added.

The 18-year-old has been charged with two counts of sexual extortion.

US detectives said the investigation was ongoing and Mr Ranaglia-Nelson could be hit with more charges if other victims came forward.

A NSW Police spokesperson said all reports of online abuse, threats and harassment are taken seriously and anyone with information in relation to child abuse material is urged to contact Crime Stoppers.