City health officials are working to track down sex partners of a man diagnosed with a rare strain of highly drug-resistant HIV that progressed rapidly to AIDS.

The virus was found in a man in his mid-40s who had unprotected sex with other men, often while using crystal methamphetamine, an addictive stimulant, health officials said Friday.

“We are not aware of another case like this in the United States, or elsewhere,’’ said Dr. Ron Valdiserri, deputy director of the Centers for Disease Control and Prevention’s National Center for HIV, STD, and TB Prevention.

Health Commissioner Dr. Thomas Frieden said the rare HIV strain is “difficult or impossible to treat.’‘

The New York Times, citing a person familiar with the case whom it did not identify, reported Saturday that the man was believed to have had unprotected sex with hundreds of people.

The man - who had not previously undergone antiviral drug treatment - was diagnosed with the rare strain in December 2004. He apparently had been infected recently after years of having unprotected anal intercourse.

The city has not released his name, but health officials are trying to locate his sex partners to offer them testing.

The onset of AIDS appears to have occurred within two to three months, and at most 20 months, after infection. Frieden said HIV can take 10 years to develop into AIDS.

Drug resistance is increasingly common among HIV-positive people, including some who had never been treated before, but not with such a fast progression to AIDS, Valdiserri said.

The man’s HIV did not respond to three of four types of antiviral drugs most commonly prescribed; he is now receiving a fourth regimen, health officials said.

Typically, drug resistance occurs after a patient is treated with the drugs, often because the patient veers from the prescribed course. In this case, resistance occurred from the start of treatment, and was combined with a rapid progression to AIDS.

Valdiserri said that “double whammy’’ of resistance and rapid progression made the case alarming.

“The message to the American public is that HIV remains a very formidable adversary. We can’t let down our vigilance,’’ Valdiserri said.

Some AIDS experts were skeptical about the announcement. Dr. Robert C. Gallo, a co-discoverer of the AIDS virus and director of the Institute of Human Virology at the University of Maryland, said some patients progress more rapidly to AIDS because they are highly susceptible, not because their strain of the virus is more deadly.

“My guess is that this is much ado about nothing,’’ he told The New York Times.

But Dr. James Braun, president of the Physicians Research Network, a New York-based not-for-profit organization of clinicians serving HIV patients, said doctors have been worried for some time about a more resistant strain of the drug.

“We believe that the transmission of treatment-resistant HIV was a disaster waiting to happen, particularly in communities where safer sex is not practiced regularly and in light of people using drugs like crystal meth,’’ he said.