The number of HIV-infected people taking anti-retroviral medicine has doubled in just five years, the UN said Monday, while highlighting high infection rates among young African women.

A new report by UNAIDS said it was on course to hit a target of 30 million people on ARV treatment by 2020.

"By June 2016, around 18.2 million people had access to the life-saving medicines, including 910,000 children, double the number five years earlier," UNAIDS said in a statement.

But the report showed the huge risks that some young women face.

Last year more than 7,500 teenagers and young women became infected with HIV every week worldwide, with the bulk of them in southern Africa.

"Young women are facing a triple threat," UNAIDS chief Michel Sidibe said at the report's launch in the Namibian capital Windhoek.

"They are at high risk of HIV infection, have low rates of HIV testing, and have poor adherence to treatment. The world is failing young women and we urgently need to do more."

Sidibe hailed progress made with HIV treatment, but warned any advance was "incredibly fragile".

The high rate of infection among young women and girls is often driven by relationships with older men, known as "sugar daddies" or "blessers".

The report said data from South Africa showed that young women and teenagers were catching HIV from adult men.

Many men catch HIV much later in life, and then continue a cycle of infection.

The report noted a study in KwaZulu-Natal province, South Africa, that revealed only 26 percent of men knew their HIV status and only five percent were on treatment.