By Alex Dobuzinskis

LOS ANGELES (Reuters) - A porn industry trade group has announced a U.S. moratorium on production of adult sex films after several reported cases of syphilis among adult film actors, adding to the pressure on porn producers to require the use of condoms on sets.

The actors can return to work in 10 days after taking antibiotics, and doctors have recommended treating all adult film actors as a precaution, the Los Angeles-based Free Speech Coalition said in a statement on its website late on Monday.

It added that filming had been halted since the weekend.

"Clearly our industry's priority is the health and well-being of our performers," Diane Duke, the executive director of the Free Speech Coalition, said in the statement.

Porn producers in the Los Angeles area -- the heart of the U.S. adult film industry and home to an estimated 1,000 performers -- are under pressure to ensure condom use on sets to prevent the spread of the AIDS virus and other sexually transmitted diseases.

Existing California workplace laws already mandate the use of condoms by porn performers, but critics say that statute is not specifically aimed at the industry and is widely flouted.

A Los Angeles ballot initiative in November will ask voters to force the county health department to require condom use by adult film performers.

Dr. Jonathan Fielding, the director of the Los Angeles County Department of Public Health, said his agency was investigating less than a dozen reported cases of syphilis involving the adult film industry.

All the cases so far involved adult film performers or their partners, Fielding said.

"It's not surprising that there would be a lot of cases in the adult film industry, since they are having a lot of unprotected sex and often with multiple partners," he said.

The AIDS Healthcare Foundation, which has led a campaign to involve local officials in regulating the use of condoms in the adult film industry, was critical of the porn industry decision to have all its performers receive antibiotic treatment, rather than take the time to trace the outbreak.

"They just want to throw an antibiotic blanket over the whole thing," AIDS Healthcare President Michael Weinstein said.

His nonprofit succeeded in January in persuading Los Angeles city officials to adopt a condom requirement for porn performers. The city is still determining how to enforce the policy, but it will not cover studio sound stages.

BALLOT INITIATIVE

A ballot initiative due in November that AIDS Healthcare supports would go further and require county health officials to impose requirements for condoms on porn sets that would apply to shoots in studio sound stages.

The Free Speech Coalition said the syphilis test that porn industry doctors were administering could not diagnose a recent infection because the test has a 90-day window.

Joanne Cachapero, a spokeswoman for the Free Speech Coalition, said her group "would rather err on the side of caution" and ensure "as many performers as possible receive testing and treatment."

(Reporting by Alex Dobuzinskis; Editing by Cynthia Johnston and Cynthia Osterman)