While the Microsoft tycoon applauded efforts to get more people worldwide on antiretroviral drugs -- and said his foundation funds both research and care -- he noted more is needed to stamp out the deadly disease.

"No one should think that we have got the tools yet. We will get the tools but only if we stay the course in terms of the scientific investments," Gates told the International AIDS Conference in the US capital.

His Bill & Melinda Gates Foundation has committed more than $2.5 billion in HIV grants to organizations around the world, and has also committed more than $1.4 billion to the Global Fund to Fight AIDS, Tuberculosis and Malaria.

Gates spoke to the conference, the world's largest gathering of AIDS scientists and advocates, as part of a panel on how to improve efficiency in funding of the three-decade-old disease that has killed 30 million people.

Gates said research toward a vaccine is "very exciting" but noted that even if a vaccine were introduced it would take time before the effects would be seen in the larger population.

"If you get a vaccine say in 10 years then the number of people you need to put on treatment is reduced in about 18 years," he said.

"Unfortunately there is this unbelievable lag time that comes out of that."

More than eight million people in low and middle income countries were on antiretroviral treatment in 2011 according to a UNAIDS report released last week, making up about half of those in need worldwide.

But Gates warned that no amount of funding can come up with enough money to treat everyone infected.

"It is clear that even if you take the most efficient way of doing this work -- the number of people who will eventually need to be on treatment, the amount of money we have is not enough to treat those people," he said.

"The world will make a decision how much those lives matter. And we are in a period of incredible uncertainty right now," he added.

"Just the uncertainty alone creates a certain instability."

Gates urged continued involvement by the AIDS community and reiterated the importance for nations and donors to support research, but also expressed support for ongoing treatment initiatives in the meantime.

"Now if somebody could cure AIDS -- which unfortunately that's very much a long shot," he said.

"There are people who are working it but... it is not in the cards at least any time soon, that is why this treatment imperative is so dramatic."

Tiny genetic mutations that make HIV immune to key frontline drugs have been increasing in eastern and southern Africa, something that should be a clear warning to health watchdogs, they said.

"Without continued and increased national and international efforts, rising HIV drug resistance could jeopardize a decade-long trend of decreasing HIV/AIDS-related illness and death in low- and middle-income countries," they said.

The study, published in The Lancet, is funded by the Bill and Melinda Gates Foundation and the European Union (EU).

It is the widest-ever analysis of a risk that has haunted AIDS campaigners since 2003, when drugs started to be rolled out to poorer countries that are home to more than 90 percent of people with the AIDS virus.

The nightmare is that -- as with bacteria which become resistant to antibiotics -- strains of HIV will emerge that will blunt the armoury of antiretrovirals, leaving millions defenceless.

Silvia Bertagnolio from the UN's World Health Organisation and Ravindra Gupta at University College London looked at published cases of HIV resistance and supplemented this with data from the WHO itself.

Over eight years, prevalance of resistant virus in untreated people soared from around one percent to 7.3 percent in eastern Africa, and from one percent to 3.7 percent in southern Africa, they found.

Similar rates of 3.5-7.6 percent were also found in western and central Africa, Latin America and the Caribbean.

The difference, though, is that they remained quite stable throughout this period, and did not experience such a big rise.

The mutations were found in strains of HIV-1 virus that made them resistant to a class of drugs called non-nucleoside reverse transcriptase inhibitors, or NNRTIs.

These are the first-option treatments for HIV infection and are also used to prevent transmission of the virus from a pregnant woman to her foetus.

If a patient is resistant to the drug, the risks of sickness and death rise in line with levels of virus.

Further treatment options do exist beyond NNRTIs, but these second-line regimens are often far costlier.

The paper says countries should step up monitoring of HIV resistance and take steps to guard against start-stop treatment that fuels the problem.

They can do this by ensuring that drug supplies are not interrupted and by beefing up monitoring of patients to encourage them to follow the daily pill-taking regimen.

Despite the concern, the rollout should carry on, says the paper.

"Estimated levels, although increasing, are not unexpected in view of the large expansion of antiretroviral treatment coverage seen in low-income and middle-income countries -- no changes in antiretroviral treatment guidelines are warranted at the moment."

Around 33 million people around the world have HIV.

In 2011, about eight million badly infected people in poorer countries had access to HIV-suppressing drugs, a figure 26 times greater than the number in 2003 but still only just over half of those in need.

The report coincided with the 19th International AIDS conference, a six-day event running in Washington until Friday.