The rate of HIV diagnoses in the United States was stable between 2001 and 2004 but fell 5 percent per year among blacks, one of the groups hardest hit by the AIDS epidemic, federal health officials said on Thursday.

The Centers for Disease Control and Prevention, which released the data, said the drop in new reported HIV infections in the black community appeared to be tied to corresponding declines among intravenous drug users and heterosexuals.

Despite the good news, officials with the Atlanta-based federal agency said the battle lines of the AIDS war continued to be marked as much by race as sexual orientation and warned high-risk groups not to be complacent about the disease.

“We must work to accelerate successes among black women and to reduce the extremely high levels of infection among black men who have sex with men,” said Dr. Ronald Valdiserri, acting director of the CDC’s national center for prevention of HIV, sexually transmitted diseases and tuberculosis.

A total of 38,685 Americans were diagnosed with HIV in 2004, compared to 41,207 in 2001, according to the CDC study, which was based on data from 33 states that have confidential, name-based reporting of HIV and AIDS cases.

Blacks, who make up about 13 percent of the population, accounted for 51 percent of the 157,252 diagnoses in the period. Their rate of HIV diagnosis was 76.3 per 100,000 in 2004, down from 88.7 per 100,000 in 2001.

Blacks were 8.4 times more likely to be told they had the AIDS virus in 2004 than whites. The U.S. rate was 20.7 diagnoses per 100,000 in 2004, compared to 22.8 per 100,000 in 2001.

The CDC said the drop in the national rate was not statistically significant.

Despite the drop in diagnoses among blacks, the overall rate has stayed steady largely because of a higher number of gay and bisexual men testing positive for the disease.

Gay and bisexual men continued to be the group most impacted by HIV, accounting for 44 percent of the diagnoses between 2001 and 2004. There were 8 percent more diagnoses in this high-risk group between 2003 and 2004.

Intravenous drug users, who can get the virus from sharing dirty needles, made up 17 percent of the total diagnoses in the three-year period, while heterosexuals accounted for about 34 percent of the new reported infections.

There was a 9 percent average annual decline in diagnoses among intravenous drug users and about a 4 percent average annual drop for heterosexuals in the period, according to the CDC study.

Most of the intravenous drug users and heterosexuals who tested positive for AIDS were black.

The agency said providing high-risk groups with testing and prevention services was a key step to halting the spread of HIV. About one-quarter of those infected are believed to be unaware of their HIV status.

AIDS activists, however, attack the approach, which was introduced two years ago, because they believe it leads to reduced funding for programs that emphasize condom use and other safe-sex practices for uninfected people.

AIDS, which destroys the immune system and leaves people vulnerable to opportunistic infections and cancers, has killed about a half million Americans and at least 22 million people worldwide since 1981.

Health experts have been warning of a possible resurgence of the epidemic, which eased in the early 1990s following the development of antiretroviral drugs targeting the disease.