"Already we are missing billions of euros in funding and the current financial crisis means that it could become more difficult to fill the gap," UNAIDS executive director Peter Piot warned at the opening of a major international conference on AIDS in Dakar.
He called on donors to keep their promises for funding to the Global Fund to Fight AIDS, Tuberculosis and Malaria.
"The fund has become an indispensible partner in the fight against HIV/AIDS and will be even more necessary in times of crisis," said Piot, who is stepping down after leading the United Nations' AIDS agency since its creation in 1995.
Burundi AIDS activist Jeanne Gapiya, who has been HIV-positive for the last 22 years, was applauded when she chided Western leaders for finding money to bail out financial firms while not meeting their obligations to the fund.
"The leaders of all the richest countries have without exception found considerable sums to fight the financial crisis," Gapiya said.
"Please stop telling us there is no more money!"
The 15th International Conference on AIDS and Sexually Transmitted Infections in Africa (ICASA) runs until Sunday.
French scientist Francoise Barre-Sinoussi, who shared the Nobel prize for medicine this year for discovering the HIV virus, told AFP that she too was concerned about the impact of the crisis.
"Like the majority of people ... we all fear the consequences of the financial crisis particularly for the engagements of countries to the Global Fund," Barre-Sinoussi said.
She stressed that assistance and HIV/AIDS funding from Western countries to the developed world have greatly helped Sub-Saharan Africa, home to two-thirds of the global total of 32.9 million people with the HIV virus.
The World Bank also fears that funding for the fight against HIV/AIDS may be hit by the financial crisis, but offered solutions for the challenges ahead.
"Historically, if we look at the 1973 oil crisis you find that there is a decline in the total official aid. So it will be very hard to avoid that phenomenon now," World Bank AIDS and economy specialist Rene Bonnel said.
"At the same time it is important to realise that there are more funding sources that can be tapped," he added.
Countries such as China, South Korea and Japan, for example, have not yet been approached for HIV/AIDS funding.
Another option is to create more cost-efficient programmes in low-income countries that are better tailored to meet those countries' needs, he added.
Elizabeth Lule, who is responsible for the World Bank's HIV/AIDS work in Africa, warned of "a very difficult, challenging period" ahead.
While those on the front lines of the AIDS battle work to fill financing gaps, other priorities were competing for donors' attention, she said.
She listed world food shortages; conflict in the Darfur region of Sudan and in the east of the Democratic Republic of Congo; Zimbabwe's cholera outbreak; and continuing problems with drug-resistant tuberculosis in southern Africa.
"The needs for more resources within these competing priorities but also with the financial crisis are enormous," she said.
"I think one concern for this conference is: will the donors and will the African government be able to manage these competing priorities and sustain the response against HIV/AIDS?"