Adult male circumcision reduces the risk of HIV infection from heterosexual intercourse by up to 60 per cent, three trials suggested.

Early results of the trials conducted in Kenya, Uganda and South Africa were so positive that the studies were ended early to give all of the men participating a chance to get circumcized.

Full data from the trial appears in Saturday's issue of The Lancet.

"This is an extraordinary development," said Dr. Kevin de Cock, director of the World Health Organization's AIDS department. "Circumcision is the most potent intervention in HIV prevention that has been described."

The studies show circumcision can reduce the risk of HIV infection in men, Marie-Louise Newell of the University of KwaZulu-Natal in South Africa and Till Barnighausen of the Harvard School of Public Health said in a commentary that accompanies the research.

If all of the 2.5 million men in KwaZulu-Natal province had been circumcised, 37,000 new infections could have been prevented in 2007, they estimated.

In circumcision, the foreskin of the penis is removed. The procedure appears to help reduce the risk of infection with HIV because without circumcision, the skin layer is thinner, allowing an easier way for the virus to enter, de Cock told the International AIDS Conference in Toronto last year.

The foreskin is also easily damaged, and its cells are especially vulnerable to the virus.

Although the data looks promising, mass circumcisions may not be appropriate, the commentary said. Circumcision carries religious and cultural issues that need to be considered, and health systems in Africa are already stressed.

While the practice reduces the risk of infection, it does not eliminate it, and circumcised men are still encouraged to wear a condom.

With files from the Associated Press