The number of adolescents dying from AIDS has tripled over the last 15 years, most of them having acquired the disease when they were infants, according to figures released Friday by UNICEF.

AIDS is the number one cause of death among adolescents aged 10 to 19 in Africa and the second leading cause of death among adolescents globally, the United Nations children's agency said in its latest statistical update.

"Among HIV-affected populations, adolescents are the only group for which the mortality figures are not decreasing," the report says.

"Most adolescents who die of AIDS-related illnesses acquired HIV when they were infants, 10 to 15 years ago, when fewer pregnant women and mothers living with HIV received antiretroviral medicines to prevent HIV transmission from mother to child."

Many of them survived into their teenage years without knowing their HIV status.

However, among teenagers aged 15-19, 26 new infections occur every hour, and about half of the two million living with HIV in this group are in just six countries: South Africa, Nigeria, Kenya, India, Mozambique and Tanzania.

"In sub-Saharan Africa, the region with the highest prevalence, girls are vastly more affected, accounting for seven in 10 new infections among 15-19 year olds," the statement said.

"It is critical that young people who are HIV-positive have access to treatment, care and support," Craig McClure, head of UNICEF's global HIV/AIDS programmes, told a conference in Johannesburg where the report was launched.

Only one in three of the 2.6 million children under the age of 15 living with HIV are on treatment.

Since 2000, nearly 1.3 million new infections among children have been averted, largely due to advances in the prevention of mother-to-child transmission of HIV.

By 2014, three in five pregnant women living with HIV received antiretroviral treatment to prevent transmission of the virus to their babies.

"This has translated into a 60 percent reduction in AIDS-related deaths among children under four years of age since 2000," Unicef said in a statement.

"These efforts to eliminate mother-to-child transmission will help to change the course of the epidemic for the next generation of adolescents."