CAMBRIDGE -- The controversy over RU-486 intensified yesterday, with feminists taking up opposite sides of the battle over whether the so-called abortion pill should be introduced in the United States.

Three researchers, all radical feminists, held a news conference at Massachusetts Institute of Technology yesterday at which they denounced RU-486 as a dangerous and cumbersome medication that should not be permitted to replace conventional surgical abortions in the United States or elsewhere.

Also yesterday, a national group called the Feminist Majority Foundation, with the backing of powerful medical associations, issued six new reports about the effectiveness of RU-486 as an abortion pill and its potential promise in treating a host of other ailments, including breast cancer and endometriosis.

The split between the groups was the latest development in a heated political battle over the abortion drug, which is vehemently opposed by antiabortion groups. Under pressure from the Bush administration, the Food and Drug Administration has banned importation of the drug for personal use here, and its French manufacturer, Roussel-Uclaf, has been reluctant to provide RU-486 even to US researchers for fear of a backlash by antiabortion forces.

Declaring that they were not joining forces with antiabortion groups, the three researchers said the current political debate over abortion has eclipsed important concerns about the drug's safety.

However, other feminist groups, along with many medical experts, say these researchers are exaggerating the dangers of RU-486. Many medical specialists and feminists consider the drug, first introduced in France and now available in Britain, to be a safe and effective drug that appeals to women because it does not involve surgery.

The Feminist Majority and powerful medical associations have launched a national campaign to bring the drug to the United States and were involved in the recent move by New Hampshire lawmakers to make the state a test site for research on RU-486.

"Withholding RU-486 from American women and women worldwide is unconscionable," said Feminist Majority's president, Eleanor Smeal. "Every major scientific association in this country supports removing nonscientific obstacles to RU-486 research."

"I think their perception is at odds with the data the French and others have collected about RU-486," agreed Dr. Philip Darney, a professor of obstretics, gynecology and reproductive sciences at the University of California, San Francisco, and is an expert on RU-486.

At yesterday's conference, the feminist scholars highlighted three major concerns about the abortion pill. First, they said, in its current usage, it also involves the administration of a synthesized prostaglandin that can have serious side effects in large doses. Second, the combined drug has a number of complications: incomplete expulsion of fetal tissue; prolonged bleeding and in some cases, vomiting and severe cramping. Third, current takers must go back to the medical clinic three or four times as part of the procedure.

"What has been missed in all the political debate is that this is not one drug, but two dangerous chemicals," said Lynette Dumble, a medical scientist at the University of Melbourne in Australia and one of the report's three coauthors. The other authors include another Australian, Renate Klein, a lecturer on women's studies at Deakin University in Victoria, Australia, and Janice Raymond, a medical ethicist at the University of Massachusetts at Amherst. One of the report's sponsors teaches at MIT.

"Prostaglandins were rejected as abortifacients in the 1970s and now they, together with RU-486, are being reintroduced to us as the abortion drug of the 90s. That's ludicrous."

RU-486 works by blocking receptors in the uterus for progesterone, a hormone that must be present for the pregnancy to continue. However, studies show that it only induces abortion in an estimated 80 to 85 percent of women, so physicians also prescribe prostaglandin, raising the rate of effectiveness to about 95 percent. Prostaglandins work by making the uterus contract and expel the fetal tissue.

To date, 65,000 French women have successfully taken this combined drug to induce abortion. Last July, however, one woman died from heart failure after taking it, a complication believed to have resulted from the prostaglandin, not RU-486.