The number of young people that have died of AIDS-related illnesses worldwide has increased by 50% since 2005 because of a lack of available treatment.
A report by UNICEF says only a third of children living with HIV in low- and middle-income countries receive antiretrovirals to keep the virus in check.
In comparison, two-thirds of adults who have the virus in the same countries have access to treatment.
There are currently 2.1 million young people aged between 10 and 19 living with HIV, whilst 210,000 children died of AIDS-related illnesses in 2012.
The organisation is calling for $1.8 billion extra funding to help 2 million adolescents, particularly girls, avoid being infected by 2020.
UNICEF Executive Director Anthony Lake said the lack of treatment was unacceptable: “Children should be the first to benefit from our successes in defeating HIV, and the last to suffer when we fall short.”
Despite the increase of AIDS-related deaths, the number of children born with HIV has halved since 2005, mainly due to improved treatments to reduce the chance of a mother passing HIV onto her child.
Countries in sub-Saharan Africa such as Ghana, Namibia and Zimbabwe saw the biggest decrease of infections in infants between 2009 and 2012.
As well as better medicines, there has been investment in better infrastructure in countries like Zambia and Malawi where mobile phones are used to quickly deliver HIV test results to give mothers a quicker diagnosis for their baby.
Michael Sidibe, Executive Director of UNAIDS, said: “This report reminds us that an AIDS-free generation is one in which all children are born free of HIV and remain so – from birth and throughout their lives – and it means access to treatment for all children living with HIV.”
Mr Lake added that it was important to act now: “It’s a matter of reaching the most vulnerable adolescents with effective programmes – urgently.”