Michel Kazatchkine told AFP during an AIDS conference in Senegal that he was "cautiously optimistic" there would not be large funding cuts despite the global economic downturn, after warnings of devastating results from campaigners.
"In my discussions with leaders up to now I have not heard of any donor that is not ready to honor their commitments to the Global Fund," Kazatchkine said.
"If the replenishment (of the fund) allows it, we will cut on the cut. The 25 percent could become 10 percent, or five percent or even zero. The cut is not a final irreversible decision."
The Global Fund to fight AIDS, Tuberculosis and Malaria, created by the G8 group of industrialized nations in 2002, accounts for one quarter of all international donations to fight AIDS.
Earlier on Saturday, non-governmental organisations warned of drastic reductions in the availability of HIV testing or medicines if the Global Fund pushed ahead with a proposed 25 percent funding cut in two years' time.
The cut would come on top of a 10-percent cost reduction in funding which the Global Fund announced it would aim for in its last round of grants.
While Kazatchkine also said he had not heard of any donors willing to increase their contributions, he said that Western governments acknowledge the importance of healthy populations to economic vibrancy.
"People have understood that health is key to development. There can be no economic growth if you do not have healthy people," he said.
"A further 25 percent cut will be disastrous," Peter Bujari of the Tanzania Health and Development Trust earlier told a press conference in Dakar at the 15th ICASA conference on AIDS in Africa, which runs until Sunday.
Bujari calculated that a 25 percent cut would lead directly to 341,000 people being denied HIV tests in 2013 in Tanzania.
"Our cry is: if money can be found to solve the credit crunch, if money can be found to save companies manufacturing toys for rich people (and) manufacturing cars, surely money can be found to fund HIV/AIDS in full," Sam Kapembwa, of the Zambian National AIDS network, added.
Kazatchkine said the current 10 percent reduction is to come from "efficiency gains."
"I do not know how difficult it is but I believe (the 10 percent reduction) will be feasible without actually decreasing the number of people getting the services at the end level," he added.