The World Health Organization on Friday endorsed the use of HIV medications among people who are not infected by the virus, but are at high risk of getting it, and encouraged both poor and wealthy countries to set up pilot projects to better understand the benefits.

The endorsement by the United Nations agency comes just four days after U.S. regulators approved the pre-exposure prophylaxis (PrEP) use of Gilead Sciences Inc.'s Truvada for healthy people who are not infected but may engage in sexual activity with HIV-positive partners.

Truvada, a 2-in-1 pill consisting of anti-HIV drugs tenofovir and emtricitabine. The antiretroviral drug that costs around $14,000 a year, which was already used to treat patients already infected with the virus, became the first treatment on Monday to also be approved for prevention.

"WHO is encouraging countries wishing to introduce PrEP to first establish small projects to help public health workers to better understand and realize its potential benefits," the agency said in a statement. WHO said appropriate HIV drugs should be given to those at high risk of infection, including uninfected men or transgender women who have sex with men who have a high risk of being HIV-positive.

The agency's spokeswoman Sarah Russell told Reuters that the WHO could not recommend specific drugs for prevention, but she said, that the drug "needs to be a drug like Truvada that has been developed for prevention purposes."

Russell said that the UN agency expects Gilead to make Truvada accessible at a deeply discounted price in some third-world countries.

"We believe Gilead will bring the price down to about $100 per year per person" in those countries, Russell told Reuters.

WHO said that its guidance is based on an international clinical study called Partners PrEP that demonstrated that a daily dose of oral HIV antiretroviral medication is highly effective at preventing the illness in men and women whose partners are infected with the virus that causes AIDS.

That study, which was funded by the Bill & Melinda Gates Foundation, was conducted in Kenya and Uganda and involved taking a daily tablet that containing either tenofovir or tenofovir in combination with emtricitabine.

The agency said that it will evaluate the outcomes of the small projects, along with the evolving scientific evidence before issuing its own detailed guidelines in 2013 on the use of antiretrovirals for HIV prevention.

To ensure that drug resistant forms of the virus do not evolve, the agency said that it was imperative that people taking HIV drugs for prevention are indeed HIV-negative.

WHO said that people who aren't infected but are at risk of contracting HIV should also continue using condoms and consistently take their medicines everyday.