The Centers for Disease Control and Prevention (CDC) said 56,000 people had become infected with the virus that causes Aids in 2006.

That is substantially more than the earlier annual estimate of 40,000.

However, the CDC said the rise was due to improved detection methods, rather than an increase in infections.

For the first time, new blood tests can tell how recently an HIV infection occurred, allowing researchers to pinpoint the year it happened.

The increase is also thought to be due to new statistical methods.
The CDC's Richard Wolitski said the 2006 incidence estimate "reveals that the epidemic is, and has been, worse than previously estimated".

The group added that the annual number of new infections was never as low as 40,000, and that it has been roughly stable since the late 1990s.

The CDC described the findings as a "wake-up call that the US HIV/Aids epidemic is far from over".

"The new estimates underscore the need to expand access to HIV prevention to gay and bisexual men, especially younger men, and to expand access to African-American men and women as well," Dr Wolitski added.

The executive director of the National Alliance of State and Territorial Aids Directors, Julie Scofield, told the Associated Press news agency: "This is the biggest news for public health and HIV/Aids that we've had in a while."

The revised estimated and the methodology behind it are due to be presented at the opening of an international Aids conference in Mexico City on Sunday.

Ahead of the meeting, thousands of activists marched through the city to protest at discrimination against people with HIV.

BBC