The White House on Wednesday expressed concern that a Texas judge could move to impose what it called a "devastating" nationwide ban on a drug used for more than half of US abortions.

Mifepristone, also known as RU-486, is one component of a two-drug regimen used for medication abortions and has been on the market in the United States for more than 20 years.

"The decision would be unprecedented, as you know, and devastating to women. And we may find ourselves in uncharted territory," White House spokeswoman Karine Jean-Pierre told reporters.

"We're closely working with the Justice Department and (the Department of Health and Human Services) on this, on how to be prepared for any range of outcomes," she added.

In a lawsuit filed in November with a Texas judge appointed by former Republican president Donald Trump, an alliance of anti-abortion groups claims the US Food and Drug Administration (FDA) should never have approved the "dangerous" drug.

Attorneys for the Alliance Defending Freedom, a Christian legal advocacy group, asked the judge to impose an immediate injunction withdrawing mifepristone from the list of FDA-approved drugs.

The FDA urged the judge to reject the request.

US District Court Judge Matthew Kacsmaryk had given parties to the federal suit until February 24 to file briefs, and so his ruling could now come at any time.

Medication abortions account for 54 percent of all abortions in the United States, according to the Guttmacher Institute, a reproductive health policy and research group.

Around 15 US states have imposed restrictions on abortion since the Supreme Court ruled in June to overturn the constitutional right to an abortion and have banned sales of the abortion pill.

But women who live in those states can still buy the pill in neighboring states, an easier option for some than a surgical abortion.