LONDON (Reuters) - The HIV/AIDS epidemic has tightened its grip on the planet, surprising experts with the speed at which it has infected 36 million people and outstripped even the worst predictions, the U.N. said on Tuesday.

More than five million new cases were reported this year alone, according to new figures released by UNAIDS, the United Nations agency that spearheads the global battle against AIDS.

"It has killed more people this year than any other year before," said Dr Peter Piot, the executive director of UNAIDS.

"Now it is clear the world has to wake up," he said in an interview. HIV/AIDS has claimed three million lives in the past two decades, UNAIDS said in its latest report.

The agency said cases of the HIV virus and AIDS were 50 percent higher than medical experts a decade ago had predicted they would be by now, despite advances in both treatments and prevention.

"The world clearly underestimated how rampant this epidemic would become," Piot told Reuters.

"We've got far more cases than the worst case scenario than thought out 10 years ago. It is the number one cause of deaths in many, many parts of the world," he said.

In Africa, the worst-hit area, infection rates have fallen slightly but only because so many people have already been struck down by it.

One million more people in sub-Saharan Africa were infected this year, a decrease from the previous year, bringing the total in the region to 25.3 million.

In some African nations one in three adults has the virus.

There has been an explosion of new cases in Russia and Eastern Europe, with the number of infections nearly doubling in just one year from 420,000 to a conservative estimate of 700,000.

New infections are on the rise in North Africa and the Middle East, the disease is gaining ground in Latin America and prevention efforts have stalled in Western Europe and North America.

Catastrophe In Sub-Saharan Africa

Africa is by far the worst hit by AIDS. It is home to 70 percent of the adults and 80 percent of the children living with HIV. It has also buried three-quarters of the more than 20 million people worldwide who have died since the AIDS epidemic began.

"The AIDS situation in Africa is catastrophic," said Piot. "One of the greatest causes for concern is that over the next few years, the epidemic is bound to get worse before it gets better."

In addition to the devastating toll of lives, the epidemic looks set to devastate African economies.

UNAIDS predicts the economy of South Africa, which has the highest absolute number of infected people in the world, could be 17 percent smaller in 2010 than it would have been without AIDS scything through its workforce.

The country's population will also shrink by 2015 and close to a third of all semi-skilled and unskilled workers will be HIV-positive by 2005.

HIV/AIDS has gained such a strong foothold on the continent for a variety of reasons including poverty, poor sanitation, crumbling health systems and sex. "You certainly won't find it in any U.N. material, but the scientific community is rapidly accepting the reality that there is more sex in Africa. There is no other affordable leisure activity," a U.N. AIDS official told Reuters.

But UNAIDS estimates that $3 billion, which is only a fraction of the $52 billion spent annually in the US on obesity, could turn the situation around. "This seems like a small price to pay to help a whole continent to avoid a future dominated by the social disruption that defines the AIDS era," the report said.

Disease With Many Faces

In Eastern Europe, Russia, Europe, North America, Latin America and the Caribbean and Asia the disease is fuelled by a combination of injecting drug users and through homosexual and heterosexual sex, the report says.

Despite improvements in antiretroviral therapy that stop the virus from replicating and intense efforts to develop a vaccine, education and prevention remain at the forefront of efforts to control the epidemic.

The head of the UN children's agency UNICEF Carol Bellamy, called for better efforts to prevent mothers from transmitting the virus to their children at birth or through breastfeeding.

Of the five million people infected with HIV/AIDS in the past year, 600,000 were transmitted from mothers to their children.

UNICEF is pushing for better screening, education and drugs to prevent babies from getting the virus from their mothers.