The UN children's fund says the number of adolescents dying from AIDS has tripled over the last 15 years, most having acquired the disease when they were infants.
AIDS is the leading cause of death among adolescents in Africa, with six countries - South Africa, Nigeria, Kenya, India, Mozambique and Tanzania - accounting for half of those living with HIV, according to UNICEF statistical data released on Saturday.
The disease is the second leading cause of death among adolescents around the world with 26 new infections occurring every hour.
In sub-Saharan Africa, seven in 10 infections in 15 to 19-year-olds are among girls, with about 70 percent of them not using condoms during sex.
Only one-third of the 2.6 million children under the age of 15 living with HIV are receiving treatment, UNICEF said.
"The issue with adolescents is that most of those teenagers dying of AIDS were infected as babies - they were infected due to mother-to-child-transmission before prevention of mother-to-child-transmission programmes were scaled up," said Craig Mcclure, global head of the HIV/AIDS section at UNICEF.
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"Also during adolescence, it's not a time you really look for HIV infection. So diagnosing adolescents living with HIV has been a problem," he said.
However, the report also said preventive measures have helped save lives, with 1.3 million new infections averted among children since 2000.
Mcclure told Al Jazeera that the rate of infection among young children has dropped by 60 percent since 2000.
"There were over 600,000 children infected with HIV in 2000 and fewer than 200,000 this year," he said.
Earlier this year, a UN report said the world can end the AIDS epidemic within 15 years.
AIDS-related deaths have dropped more than 40 percent since 2004 to 1.2 million a year, the report said, adding that new HIV infections have fallen 35 percent since 2001 to two million a year in 2014.