"But if we relax now, it would be disastrous. It would wipe out all of our previous investments."
The annual assessment on the state of the global epidemic said the spread of the disease appeared to have stabilised after peaking in the 1990s.
The agency estimates there are now 33 million Aids cases worldwide.
More on medication
The most dramatic figures were seen in the number of patients on Aids medication, which registered a ten-fold jump in the last six years.
In 2007 about 3 million people were taking Aids drugs compared to some 300,000 in 2003, mostly because medication has become much cheaper and more available.
Officials estimate that 2 million people died from Aids last year, down from approximately 2.2 million in 2005.
But UNAids said the battle was far from won as the epidemic remained the leading cause of death in Africa, home to almost 70 per cent of the world's Aids sufferers.
Countries in sub-Saharan Africa including South Africa, Botswana and Swaziland remain the centre of the epidemic.
The region has about 67 per cent of all people infected with HIV and 72 per cent of all Aids deaths.
The UNAids report based on government data from 147 countries also warned that there could be future waves of infection, adding that it would be difficult to predict whether the epidemic might spike again.
Aids in Asia
|The annual report also warned of Aids spreading beyond Africa [EPA]
Outside sub-Saharan Africa, Aids mainly affects drug users, gay men and sex workers, the report says.
It also said the number of cases beyond Africa was on the rise, including in China, Germany, Indonesia, Russia and Britain.
According to Piot of UNAids, there is a high HIV infection rate among gay men in many parts of Asia, saying the rate of infection was "of the same magnitude" as seen in the US 25 years ago".
He added that there was still not enough action in the region to combat the rise in infections because the cases were detected fairly recently.
Commenting on the UNAids report, Son Gwi-Yeop, the UN coordinator and development representative in Thailand, said high investments made in prevention work had yielded "some positive results".
"But the challenges remain quite large as we have a lot of cases in Asia and HIV/Aids is far from over in the region," she told Al Jazeera.
"We need a lot more investments to ensure that we are able to tackle this challenge positively."
Last week, the US announced it would triple its funding for Aids and related diseases around the world to $48bn over the next five years.