Circumcising men can help protect them from the AIDS virus, researchers said Tuesday after finishing the first study that tried using the procedure specifically to prevent infection.

But United Nations health officials cautioned that more trials were necessary before they would recommend this as a method to protect against AIDS.

The circumcised men were 65 percent less likely to become infected with the deadly and incurable virus, the researchers told the International AIDS Society Conference in Rio de Janeiro .

Several studies have suggested that men who are circumcised have a lower rate of HIV infection. This has been especially noticeable in some parts of Africa , where some groups are routinely circumcised while neighboring groups are not.

Researchers believe circumcision helps to cut infection risk because the foreskin is covered in cells that the virus seems able to easily infect. The virus may also survive better in a warm, wet environment like that found beneath a foreskin.

Bertran Auvert of the French National Research Agency INSERM and colleagues tested more than 3,000 uninfected men aged 18-24 South Africa 's Guateng province, where 32 percent of adults are infected.

Some of the men were circumcised at the start of the trial while others had the procedure after 21 months. At the end of the trial, researchers registered 69 HIV cases, and only 18 of them were in the men who were circumcised at the start.

"It shows that the intervention (circumcision) prevented between six and seven out of 10 possible infections," said Auvert, noting that although the protection rate was high it was only partial.

Promising trial

He said the results were consistent with earlier simple observational studies in some African countries and India .

"Although the trial shows promising protective effects of adult male circumcision in reducing HIV acquisition, UNAIDS emphasizes that more research is needed," the United Nations AIDS agency said in a statement.

Two more studies underway in Uganda and Kenya will provide more evidence, UNAIDS said.

"It is a very sensitive issue, and not just biologically," said UNAIDS chief scientific adviser Catherine Hankins, adding that circumcision should only be used as part of a package of preventive measures such as the use of condoms and behavioral changes to limit the number of sexual partners.

The World Health Organization said it was working on guidelines for qualified medical personnel to conduct safe circumcision as demand for the operation may soon increase.

Health officials said they were concerned that such demand may increase the number of operations carried out by healers and witch-doctors, which could boost the risk of HIV infection rather than prevent it if they were not done correctly.