Reports of rare bacterial infections, including a handful of cases in women who have taken the controversial RU-486 abortion pill, are cause for concern and warrant further study, U.S. health experts said on Thursday.

Two sometimes fatal bugs - Clostridium sordellii and Clostridium difficile - are a particular worry as antibiotic resistance grows and more infections occur in people without typical risk factors, doctors and researchers said.

While the infections also have been reported in drug users, surgical patients and accident victims, including men, those found in women taking the pill drew the most scrutiny at a public meeting at the Centers for Disease Control and Prevention’s Atlanta headquarters.

Officials from the CDC, the Food and Drug Administration and the National Institutes of Health are seeking advice from outside experts to decide what research is needed to understand and track the infections.

Paul Seligman, associate director for safety policy at the FDA, said it was unclear what was triggering the new clusters of cases.

“What we do know is that in this country we are seeing the simultaneous emergence of two virulent, often fatal illnesses affecting otherwise healthy people,” he said at the meeting.

Drawing the most scrutiny are at least 15 pregnancy-related cases, including 10 fatal infections reported recently in women who had given birth or who had miscarriages.

Another six women who took RU-486, also known as Mifeprex or mifepristone, have died since 2000. Officials linked four of the cases to infection but could not tie them to the drug. Another case is under investigation and another was ruled unrelated.

Representatives of several women’s groups and others who support RU-486 have said the infections were important to investigate, while anti-abortion groups saw the meeting as a first step to ban the pill.

Dozens of U.S. lawmakers, mostly Republican, back legislation calling for the drug’s withdrawal, and anti-abortion groups also have petitioned the FDA.

The drug, made by Danco Laboratories LLC, is taken with another drug called misoprostol early in pregnancy to trigger an abortion. It is unrelated to emergency contraception sold by Barr Pharmaceuticals Inc. under the name Plan B.

While abortion supporters and opponents attended the largely academic meeting, it was free of fervent protest often found at events that touch on abortion issues.

Instead, agency officials focused on scientific evidence.

FDA’s Seligman said thousands of Clostridium difficile cases occur each year in the United States. The diarrhea-causing disease is usually treatable but has recently become more difficult to treat.

Clostridium sordellii is far more rare and previously was not known to be toxic. “Over the past few years the picture has changed,” Seligman told the panelists.

Two experts singled out the abortion pill as a problem. University of Colorado Health Sciences Center gynecologist James McGregor urged officials to “reduce or eliminate” use of Mifeprex.

Overall, most experts from state public health offices, hospitals and universities encouraged further study.

“We clearly need controlled trials,” said Dale Gerding, an associate chief of staff at Hines Veterans Affairs Hospital in Illinois.

It was not immediately clear what future action the FDA might take regarding RU-486 or antibiotic use.

All three agencies will carefully weigh the information presented, Seligman said, adding that “developing a realistic set of expectations” about how long it will take to understand the infections would play a part in deciding what, if any, action the FDA might take.”