The research, presented at the International AIDS Conference in Washington on Monday, offers stronger evidence to existing estimates that young black men who have sex with men represent the leading edge of the U.S. HIV epidemic.

The team said the overall infection rates among U.S. black gay and bisexual men rival those seen in sub-Saharan African countries that are hardest hit by the virus that causes AIDS.

"I'm shocked and I'm worried," said Dr. Carlos del Rio, a researcher at Emory University in Atlanta, who led the study. "It's a public health emergency. It is something we as a nation should not allow to happen," he said.

The study, conducted between 2009 and 2011, enrolled a total of 1,553 gay and bisexual men from six cities: Atlanta, Boston, New York, Los Angeles, San Francisco and Washington, D.C.

Most of the men identified themselves as black, del Rio said, and while not all identified themselves as gay or bisexual, they were having unprotected sex with men.

The team tested all of the men for HIV infection, and referred those who were infected to get treatment.

They continued to follow those who were not infected for a year, and found that overall, 2.3 percent of the men in the study became infected with HIV, a rate 50 percent higher than white men who have sex with men.

The numbers were more startling when the researchers looked at men under the age of 30. In these men, 5.9 percent were infected with HIV at the end of the study period, a rate three times higher than white men who have sex with men.

The study also highlighted the problem of high infection rates and low testing rates. Of the men in the study who had not been tested for HIV or did not know their status, 12 percent had positive tests at the start of the study.

The six-city study was funded by the National Institutes of Health.

In a separate ongoing study by researchers at Emory looking at 709 men who have sex with men in Atlanta, researchers found that while both blacks and whites had comparable levels of risky sexual behaviors, blacks in the group were more than twice as likely to be infected with HIV as whites.

"Differences in individual risk behavior don't explain the large disparities in HIV prevalence and incidence between black and white MSM (men who have sex with men)," Eli Rosenberg, one of the study's authors, said in a statement.

Rosenberg said strategies to address these disparities needed to include increased HIV testing and treatment of all men who test positive with HIV drugs, which have been shown to cut the risk of transmitting HIV to a sexual partner by 96 percent.

(Reporting by Julie Steenhuysen; Editing by David Brunnstrom)

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