The UN agency tasked with fighting AIDS on Tuesday welcomed the decision by the United States to allow the use of an HIV prevention pill for the first time.

The pill will be used "to prevent the sexual transmission of HIV for people who do not have HIV, but who are at high risk of being exposed to the virus", UNAIDS said in a statement from its Geneva headquarters.

Since 2004, the combination of drugs contained in the pill -- emtricitabine and tenofovir disoproxil fumarate -- has been used to help those with HIV, but not to protect against infection, the agency said.

The US Food and Drug Administration (FDA) approved the pill -- known as Truvada -- on Monday based on results from clinical trials which found that a daily oral dose of tenofovir, or emtricitabine/tenofovir disoproxil fumarate, carried significant preventative benefits, UNAIDS said.

HIV prevention of this kind is called pre-exposure prophylaxis, or PrEP, which the FDA approved as part of a wider HIV prevention strategy that includes condom use and other safer sex practices, risk reduction counselling and control of other sexually transmitted infections.