Gay men practicing risky sexual activity are fueling a sharp increase in the incidence of syphilis and a smaller, albeit concerning rise in gonorrhea resistant to commonly used antibiotics, federal researchers said this week.

The latest report of increases comes at a time when STD rates among historically important risk groups (including women and minorities) have been declining, according to a new report from the Centers for Disease Control and Prevention.

These diseases “are a significant and ongoing threat to millions of Americans,’” said Dr. Ronald Valdiserri, acting director of CDC’s National Center for HIV, STD and TB Prevention.

Health authorities made major inroads into syphilis transmission during the ’90s, bringing the annual number of new cases during 2000 to the lowest level since record-keeping began in 1941.

But from 2000 to 2004, the rate of infection rose 29 percent to a total of 7,980 cases, with most of the increases occurring among men. Men engaging in homosexual activities accounted for 64-percent of syphilis infections in 2004, compared to about 5 percent in 1999, said John Douglas, director of the CDC’s division of STD prevention.

The number of cases is relatively small, but the data is potentially alarming, experts said, because increases in STD’s are assumed to be precursors of increases in the transmission of HIV.

San Francisco had the highest incidence of syphilis: 45.9 cases per 100,000 people.