WASHINGTON (AP) _ The Justice Department told the Supreme Court today that federal judges have no authority to overturn the Food and Drug Administration's ban on the French abortion pill RU486.

''The FDA's decision to enforce the indisputable terms of a concededly valid statute against an unmistakably clear violation is not the proper subject of judicial review,'' the government said in a 13-page brief submitted at the request of Justice Clarence Thomas.

As the justice responsible for emergency case arising in New York, Thomas has been handed the hurry-up appeal of a test case involving a woman challenging the U.S. ban on the pill.

Leona Benten, a 29-year-old American social worker who brought RU486 from England on July 1, is nearly eight weeks pregnant and can safely start taking the pills only until Saturday, her lawyers say.

Customs officials seized the pills when the woman arrived at New York's Kennedy International Airport. They still have them. She has sued to get them back.

Abortion rights groups had arranged Ms. Benten's trip and alerted Customs to her arrival to challenge the U.S. ban on the pill. They charge it stems from the Bush administration's anti-abortion stance.

But the Justice Department said the government was merely enforcing ''the unambiguous command of the Food, Drug and Cosmetic Act, which forbids importation of unapproved drugs such as RU486.'' The act requires that companies seeking approval for a drug must submit data proving safety and effectiveness. That has not been done in the United States.

A U.S. District judge, a Democrat appointed by President Carter, ruled in her favor based on the FDA's policy of allowing certain unlicensed drugs into the country. The 2nd U.S. Circuit Court of Appeals stayed his order. Two of the three circuit judges were appointed by Presidents Reagan and Bush, Republicans who oppose abortion. Ms. Benten took the case to Justice Thomas.

The Justice Department acknowledged to Thomas that FDA, primarily to conserve manpower, had exercised its discretion to allow some unlicensed drugs brought in ''by individuals for personal use.''

The government said these fell into two categories ''innocuous drugs acquired abroad for treatment of non-serious conditions'' and ''drugs of unproven efficacy for serious conditions for which no effective treatment exists.''

In the first category, the government listed headache remedies, anti-itch and anti-wrinkle creams. In the second category, it listed drugs used to treat AIDS and certain cancers.

The government said Ms. Benten had a legal alternative, surgical abortion, and she ''does not allege that a surgical abortion would be ineffective or unusually unsafe in her case.''

The government said the district judge erred in ''arrogating to itself and role of supervisor of the public's health and safety.'' It endorsed the stay issued by the circuit court.

In interviews, FDA officials have said no company has asked for permission to market the abortion drug in the United States.

''It's not that the FDA has refused to approve it,'' said agency spokesman Gary Fendler. ''We can't answer a question that we haven't been asked.''

And, the agency doesn't look upon pregnancy as fitting the loophole it created to allow people faced with life-threatening illnesses to bring in unapproved drugs for their own use.

''We do allow in very limited situations unapproved drugs into country for personal use for serious and life-threatening diseases for which there is no known cure or therapy,'' Fendler said.

RU486, developed by the French pharmaceutical company Roussel Uclaf, causes the uterus to shed its lining and expel the developing embryo. It has been used by some 110,000 women in Britain and France.

Fendler said RU486 was tested in the United States in the mid-1980s but the company, which he did not name, did not then submit a request for approval to market the product.