Image caption The UN says young people are becoming sexually active later

The prevalence of HIV among young people in countries worst-affected by Aids, mainly in Africa, has fallen, new figures from the UN show.

In a report, UNAids says the incidence of HIV has decreased by up to 25% as young people between the ages of 15 and 24 change their sexual behaviour.

The report says it is in response to Aids prevention campaigns.

But the UN says it is on the rise in Uganda, which had been praised for its HIV fight, because of "complacency".

According to the UN, five million young people live with HIV worldwide, making up 40% of new infections.

'Warning'

Uganda's vigorous campaign against HIV/Aids had helped to reduce the prevalence of the virus - which reached 30% in the 1990s - to single-digit figures.

Young people are not just perceiving themselves anymore as a passive beneficiaries of programmes, but they are making themselves actors of change Michel Sidibe, Executive director, UNAids

"After the reduction and introduction of treatment, most of the people were not feeling anymore of the same pressure for prevention programmes," Michel Sidibe, the executive director of UNAids, told the BBC.

"So what we are experiencing today in Uganda is what we need to be scared about it - it's progress, and not sustaining [those] results due to probably a complacency."

However, the other data was a positive sign of change as young people in Africa were taking responsibility for their own health and well-being, he said.

"Young people are not just perceiving themselves anymore as a passive beneficiaries of programmes, but they are making themselves actors of change," he said.

"For me, that is a major, major shift in our prevention programmes."

Treatment problems

The BBC's Imogen Foulkes in Geneva says the UN is releasing the figures ahead of this year's international conference on Aids, which begins in Vienna on Sunday.

Case study: Kenya 60% decline in HIV prevalence between 2000 and 2005

Urban areas: HIV prevalence dropped from 14.2% to 5.4%

Rural areas: HIV prevalence dropped from from 9.2% to 3.6% Source: UNAids

The Outlook report says young people in 16 of the world's 25 worst-affected countries with HIV are becoming sexually active later and having fewer sexual partners.

In countries such as Botswana, Ethiopia, Kenya, Malawi and Zimbabwe the reduction in new HIV infections, measured among young pregnant women presenting for antenatal check-ups, indicates that these nations will achieve UN targets for reducing HIV rates among the young this year.

While the UN believes significant progress is being made, on treatment the picture is somewhat different, our reporter says.

Aids treatment remains complicated and expensive, and, worldwide, only a third of those who need anti-retrovirals are actually receiving them.

The UN says more resources are needed to develop simpler and cheaper Aids medicines, and to streamline diagnosis and treatment.