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Twenty-five years after the first AIDS cases jolted the world, US scientists think they soon may have a pill that could prevent HIV, the virus that causes the global killer.

"This is the first thing I've seen at this point that I think really could have a prevention impact," said Thomas Folks, head of the HIV research lab at the US Centers for Disease Control and Prevention. "If it works, it could be distributed quickly and could blunt the epidemic."

Two drugs already used to treat HIV infection have shown such promise at preventing it in monkeys that officials last week said they would expand early human tests around the world.

The drugs are tenofovir (Viread) and emtricitabine, or FTC (Emtriva), sold in combination as Truvada by Gilead Sciences Inc, a California company best known for inventing Tamiflu, a drug showing promise against bird flu.

If larger tests show the drugs to be as effective on humans, they could be given to people at highest risk of HIV.

Condoms and counselling alone have not been enough - HIV spreads to 10 people every minute, five million every year. A vaccine remains the best hope but none is in sight.

Matthew Bell, a 32-year-old hotel manager in San Francisco who has volunteered for a safety study on one of the drugs, said he would welcome taking a drug as an added precaution to practising safe sex.

"As much as I want to make the right choices all of the time, that's not the reality of it," he said of practising safe sex. "If I thought there was a fallback parachute, a preventative, I would definitely want to add that.