There is evidence of an increase in unsafe sexual practices by men who have sex with other men in the US, according to the annual UN Aids report.
By Peter Bowes
BBC News, Los Angeles
The report's section on the US also focuses on the growing number of black women becoming infected with HIV.
The number of people living with HIV in the US has reached its highest level ever, put at 1.2 million in 2005.
The report says widespread access to the latest drugs has kept the number of Aids-related deaths relatively low.
About 16,000 people died last year, but preventing new infections remains a challenge.
Cause of death
Half of all HIV infections diagnosed in 2004 were men who have sex with men - and the report says there is evidence of resurgent risky behaviour in this group.
Public health officials in major US cities confirm that the perception that HIV is no longer a serious problem may have resulted in some gay men ignoring safer sex practices.
The UN report says Aids is the leading cause of death among black women aged 25-34.
African-Americans make up about 12% of the population of the US and account for 50% of all new HIV cases.
The report points out that black women are half as likely to be receiving the latest drug therapy compared with other population groups.
In Los Angeles, public health workers on the ground say poverty, a lack of education about safe sex and intravenous drug use are all to blame.