PHNOM Penh's Municipal Court on Tuesday charged a US national with purchasing sex from an underage girl, a court prosecutor told the Post.

Deputy prosecutor Sok Kalyan, who filed the charges, said the 49-year-old man remains detained and faces seven to 15 years in jail if convicted. He said his case had been forwarded to an investigating judge.

The man, identified by Samleang Seila, director of the anti-paedophile NGO Action Pour Les Enfants as Boyasian Gerard, was arrested last Thursday by police from the Interior Ministry's Department of Anti-human Trafficking and Juvenile Protection in Svay Pak, a once-notorious child sex district 11 kilometres outside Phnom Penh. He was found engaging in sexual acts with an eight-year-old Vietnamese girl, Samleang Seila said, adding that the man is believed to have solicited sex with more underage girls there.

The self-contained brothel village catering to foreigners and Khmers had been shut down in a number of high-profile police raids in the past.

"[Gerard] had fled from his country to live in Cambodia," Samleang Seila said. "He had been convicted in his own country of a similar charge."

Gerard's defence lawyer, Ouk Saroeun, could not be contacted for comment on Wednesday.

After years of being seen as a haven for sexual predators, the government began a campaign to clean up the Kingdom's image in 2003, arresting paedophiles and other sex offenders including dozens of foreigners.