A fourth American porn actor has reportedly tested positive for HIV, fuelling fears of a significant outbreak in California's adult film industry.

A male performer told a Los Angeles-based advocacy group he had been diagnosed with the virus that causes Aids, it emerged on Monday, following three other cases in recent weeks.

"I can confirm that a male performer came to us and indicated that he had become infected with HIV," said Michael Weinstein, president of the Aids Healthcare Foundation.

The disclosure will likely pressure the industry to continue a moratorium on production and bolster calls for mandatory use of condoms. "It's Russian roulette. I don't know how many people have to be infected before things change," Weinstein said.

Citing confidentiality, he declined to identify the latest performer to come forward or to elaborate on his circumstances.

A 29-year-old female actor who uses the screen name Cameron Bay was the first to go public in August, saying she tested positive after a routine HIV test.

The Free Speech Coalition, a Los Angeles-based trade organisation for the industry, called for a week-long suspension of filming pending tests for other performers.

On 3 September, soon after the moratorium was lifted, a male performer who uses the screen name Rod Daily and has been romantically linked to Bay announced via Twitter he had the virus. "Drumroll please!! I'm 32 years old and I'm HIV positive. Acute HIV, which means I recently was infected." A test one month earlier had been negative, he said. "I'm blessed for the fact that I caught it so early that I can blast that shit with meds."

The Free Speech Coalition reinstated the moratorium on 6 September when an industry-affiliated doctor contacted it to say a third performer had tested positive. The fourth case has not been officially confirmed.

Weinstein said questions over whether the virus was transmitted on or off set, and the duration and enforcement of moratoriums, distracted from the fundamental issue. "What works is condoms."

The industry, which is based in the San Fernando Valley area of Los Angeles, has resisted efforts to impose condom use, saying it would kill the business and that regular screening protects performers. The Free Speech Coalition did not immediately respond to news of the latest case. It has said it is considering making tests for sexually transmitted diseases more frequent, from 28 to 14 days.

Last year voters approved a measure mandating condoms for productions in Los Angeles county but enforcement has been lax. A bill that would mandate condoms in porn productions across California has stalled in Sacramento.