There is not enough evidence to say circumcision protects men from getting the AIDS virus during sex with other men even as studies show it protects them when having sex with women, U.S. researchers said on Tuesday.

A review of 15 studies involving 53,567 gay and bisexual men in the United States, Britain, Canada, Australia, India, Taiwan, Peru and the Netherlands failed to show a clear benefit for those who were circumcised, researchers from the U.S. government's Centers for Disease Control and Prevention said.

Circumcised men were 14 per cent less likely to be infected with the human immunodeficiency virus, or HIV, than those who were uncircumcised, but the finding was not statistically significant, the CDC researchers said.

"You can't necessarily say with confidence that we're seeing a true effect there," said the CDC's Gregorio Millett, who led the study that appeared in Wednesday's Journal of the American Medical Association.

"Overall, we're not finding a protective effect associated with circumcision for gay and bisexual men," Millett said in a telephone interview.

Studies involving men in Africa, where the AIDS epidemic is primarily spread by sex between men and women, showed that male circumcision halved the risk of female-to-male HIV infection.

Experts say this reduced HIV risk may be because cells on the inside of the foreskin, the part of the penis cut off in circumcision, are especially susceptible to HIV infection. The virus also may survive better in a warm, wet environment like that found beneath the foreskin.

But whether circumcision might lower the risk of HIV infection in sex between men had remained unclear. Gay and bisexual men play a much larger role in AIDS in many countries outside of Africa, the epidemic's epicentre.

Not recommended strategy

For example, the CDC last week said 48 per cent of the 1.1 million Americans infected with HIV are men who have sex with men. More than three-quarters of U.S. men are circumcised.

"We really cannot recommend overall male circumcision as a strategy for men who have sex with men in the United States," Millett said.

The CDC's Dr. Peter Kilmarx, who was not involved in the research, said the agency is preparing formal recommendations on circumcision in the United States, with a draft due to be made public early next year.

Millett said there are signs circumcision might protect certain gay and bisexual men depending on sexual practices.

The virus can be transmitted through blood or semen.

Studies in Australia and Peru showed that men who engaged in insertive anal sex only and were not being penetrated by male sex partners got a significant protective effect from HIV infection from being circumcised, Millett said.

"Of course, if you're being penetrated by a partner during sex, you being circumcised is not going to protect you from HIV infection," Millett said.

Millett said two U.S. studies and one in Peru conducted before the introduction in 1996 of combination drug treatment for HIV infections, called highly active antiretroviral therapy, or HAART, showed that circumcised men were 53 percent less likely to be infected with HIV than uncircumcised men.

He said it is possible that since the advent of HAART, which helped turn HIV infection into a chronic disease rather than a death sentence for many people, some gay and bisexual men may have felt freer to engage in risky sexual practices.

With files from Reuters