Surveillance for the HIV virus is weak in most of the world and prevention and treatment programmes often fail to reach high-risk drug users, homosexuals and sex workers, the World Health Organization said on Friday.

In a message marking World AIDS Day, being celebrated under the theme of Accountability, the WHO's Acting Director-General Anders Nordstrom said that tackling the AIDS epidemic remained one of the world's most pressing public health challenges.

Only 1.6 million people or 24 per cent of the 6.8 million people worldwide who need the life-extending therapy receive it, according to the latest joint report of UNAIDS and the WHO.

"We have a very long way to go still in the provision of medicines to those who need them," the WHO's Nordstrom said.

"HIV surveillance remains weak in almost all regions, particularly among marginalized groups. Those at highest risk men who have sex with men, sex workers and injecting drug users are not reliably reached through HIV prevention and treatment strategies," he said.

People most at risk of exposure to the deadly virus do not always know how to protect themselves and often lack access to condoms, clean needles and syringes, according to Nordstrom.

"Even in countries where the epidemic has a very high impact, such as Swaziland and South Africa, a large proportion of the population do not believe they are at risk," he added.

Nearly 40 million adults and children are infected worldwide and HIV infection is rising in every region of the world, especially in East Asia and in Eastern Europe/Central Asia, according to a UNAIDS/WHO report.

Louise Arbour, UN High Commissioner for Human Rights, reminded governments of their duty to increase access to anti-retroviral drugs for all AIDS sufferers, without discrimination.

"It means holding governments accountable for obligations of immediate effect, for example where scaling-up access (to the drugs) discriminates against a certain group such as children, those involved in the sale of sexual services or injecting drug users," Arbour said in a statement.

UN Secretary-General Kofi Annan said AIDS has become the greatest challenge of the generation, but for far too long the world had been in denial about the epidemic which emerged 25 years ago.

"Accountability the theme of this World AIDS Day requires every president and prime minister, every parliamentarian and politician, to decide and declare that 'AIDS stops with me'," Annan said.