Science has given the world "no excuse" to resist bold action against the spread of the 30-year AIDS pandemic, said a top US expert at the opening of the International AIDS Conference on Sunday.

This year's meeting, themed "Turning the Tide Together," is the world's largest gathering on HIV/AIDS and is expected to draw 25,000 people, including politicians, scientists, celebrities and activists.

High-profile guests at the six-day event include Secretary of State Hillary Clinton, who will speak on Monday at 1400 GMT, former president Bill Clinton, former first lady Laura Bush, singer Elton John and actress Whoopi Goldberg.

President Barack Obama will not attend in person but will send a video message and will invite some attendees to the White House for talks on Thursday, a top health official said.

Many experts have spoken of their optimism in the days leading up to the event. But Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said the pace of progress must accelerate.

"The fact remains that right now, today, in the summer of 2012, 31 years after the first cases were reported, there is no excuse scientifically to say we cannot do it," Fauci said.

"What we need now is the political, organizational and individual will to implement what science has given us."

Fauci cited the recent UNAIDS report released last week that showed there were 2.5 million new infections worldwide in 2011, down from 2.7 million in the prior report.

"But that makes the slope look like that," he said, holding out a flat hand. "That is unacceptable."

Among the tools that scientists have are trials that show treatment can reduce the risk of transmitting the virus to others, and antiretroviral drugs that, when taken by healthy people, can help reduce their risk of becoming infected.

Also on opening day, an international group of scientists called for all adults who test positive for HIV to be treated with antiretroviral drugs right away rather than waiting for their immune systems to weaken.

The International Antiviral Society is the first global group to make such a call.

Other major groups, such as the World Health Organization, currently urge treatment after the disease progresses to a certain point, or when the body's T-cell count, or CD4 count, reaches or falls below the level of 350 cells/mm3.

"These guidelines are aspirational," said Melanie Thompson, a doctor with the AIDS Research Consortium of Atlanta, urging increased testing and better care of those who test positive for HIV.

"The science should drive the allocation of resources and guidelines can play an important role in that respect."

Some 34 million people in the world are living with HIV, according to the latest UNAIDS report issued last week. However, about one in five people are not aware of their status and are most at risk of spreading the disease.

Some eight million people in low and middle income countries are now on antiretroviral treatment, making up about half of those in the world who need it, the UNAIDS report said.

Held every two years, the conference has returned to the United States for the first time since 1990, after being kept away by laws that barred people with HIV from traveling to the country. The US ban was formally lifted in 2009.

The hunt for a cure, which has eluded scientists, will be another hot topic. HIV co-discoverer and Nobel laureate Francoise Barre-Sinoussi announced on Thursday a new roadmap for scientists in research toward a cure.

The only man who has achieved a functional cure of HIV though a bone marrow transplant, American Timothy Brown, also known as the "Berlin patient," will address the conference on Tuesday at 1500 GMT to publicize new efforts in this direction.

Funding, too, is at a critical juncture, with many nations boosting their domestic spending on the disease while international donations remain flat.

Total worldwide investment in HIV was $16.8 billion last year, an 11 percent rise from 2010, but still far short of the $22-24 billion needed by 2015, according to UNAIDS.