Aids is spreading faster than ever, outstripping efforts to contain it, UN Secretary General Kofi Annan has said.
Mr Annan is calling for more money and better leadership
"Last year saw more new infections and Aids-related deaths than ever before," Mr Annan told a conference in New York.
Only 12% of people with Aids in underdeveloped countries are getting anti-retroviral drugs, he added.
In 2001, the UN set a target date of 2015 to halt the spread of the disease but Mr Annan said better leadership and funding was needed to reach that goal.
"The fight against Aids may be the great challenge of our age and our generation," Mr Annan told delegates.
"Only if we meet this challenge can we succeed in our efforts to build a humane, healthy and equitable world.
FACTS ABOUT HIV/AIDS
More than 39m HIV sufferers around the world
In 2004, HIV/Aids killed more than 3m people
Sharpest rise in HIV recorded in East Asia, eastern Europe and Central Asia
"Let us ensure we are equal to that task."
The one-day meeting - set up to assess progress on the UN's targets - was attended by delegates from 120 countries.
Mr Annan praised Brazil for having the most successful Aids programme in the world.
Cambodia and Thailand have implemented prevention programmes and shown substantial progress, he said. But many more countries still need to develop action plans.
"We are still moving into a globalisation of the Aids epidemic - think of eastern Europe, central America, Asia and maybe tomorrow, the Middle East as well," said the director of Unaids, Peter Piot.
He urged world leaders to give Aids the same attention they devote to global security.
In a report published to coincide with the conference, Mr Annan warns that targets, such as cutting HIV infections in young people by 25% by 2005, will not be achieved.
However, his report says funding for Aids work in developing countries has increased from $2bn in 2001 to about $8bn in 2005, although this still falls short of the resources the UN believes are needed to properly tackle the epidemic.
The number of women accessing services to prevent mother-to-child HIV transmission has increased by 70%, and the number of young people who have received Aids education has doubled, the report says.
It warns that, at the end of 2004, only 12% of the six million people who need HIV treatment worldwide had access to it and only one in five people across the world has access to prevention services.
Services aimed at preventing HIV infections have only reached a small proportion of people in high-risk groups such as sex workers, men who have sex with men and street children and intravenous drug users.
World leaders are due to meet at the UN in September to discuss the 2015 target, one of eight UN Millennium Development Goals which all member countries signed up to.
"The task this year will be much tougher than in 2000 when the declaration was adopted," Mr Annan said.
"Instead of setting targets, this time leaders must decide how to achieve them."