LOS ANGELES (Reuters) - A porn industry trade group has announced a U.S. moratorium on production of adult sex films after at least one Los Angeles actor tested positive for syphilis, 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.

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, which is 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 require the county health department to require condom use by adult film performers.

The Los Angeles County Department of Public Health said in a statement that it was "investigating recent reports of possible cases of syphilis in the adult film industry."

It was unclear exactly how many porn actors have tested positive for syphilis. The Free Speech Coalition said on its website that a single performer had tested positive for the disease.

But Peter Kerndt, the director of the county's sexually transmitted disease programs, told the Los Angeles Times in an article posted on its website on Friday that his agency had received reports of at least five possible cases.

Sarah Kissel, a spokeswoman for the Department of Public Health, declined on Tuesday to say how many cases had been reported to the county.

The nonprofit AIDS Healthcare Foundation has led a campaign to involve local officials in regulating the use of condoms in the adult film industry. In January, it succeeded 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.

A ballot initiative in November, which resulted from a signature-gathering drive by the AIDS Healthcare Foundation, would require county health officials to impose a more far-reaching requirement for condoms on porn sets, and that would apply to shoots in studio sound stages.

The Free Speech Coalition last year called for a temporary moratorium on filming after a performer tested positive for HIV. The actor was later found to not have the virus.