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, US 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 US government's Centers for Disease Control and Prevention said.

Circumcised men were 14 percent less likely to be infected with the human immunodeficiency virus, or HIV, than those who were not 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 the 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 epicenter.

For example, the CDC last week said 48 percent of the 1.1 million Americans infected with HIV are men who have sex with men. More than three-quarters of US 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.