Two U.S. nuns who signed a newspaper advertisement disagreeing with the Roman Catholic Church's stance against abortion were threatened today by the Vatican with disciplinary action unless they renounce their dissent.

The Vatican said it has "accepted public declarations of adherence to Catholic doctrine on abortion" from the other 25 members of the clergy -- nuns, priests and brothers -- who, along with 70 lay Catholics, signed the advertisement published during the 1984 U.S. presidential election campaign and now "considers their cases closed." Some of the 22 nuns cleared in the controversy, however, have denied that they made such declarations.

The warning to Sisters Barbara Ferraro and Patricia Hussey, of the Notre Dame de Namur order, came in a statement by Cardinal Jerome Hamer, prefect of the Vatican's Congregation for Religious and Secular Institutes, which oversees the church's religious orders.

"Two sisters of Notre Dame de Namur refuse to state that they accept the authentic teaching of the Catholic Church on abortion, in spite of initiatives and invitations from the Holy See," Hamer said.

His statement said the Vatican was "closely following the efforts of the general government of the Notre Dame sisters with the hope that the cases of their two sisters will be resolved without having to begin formal disciplinary procedures in accord with church law."

The statement was seen by Vatican experts here today as a clear threat by Hamer to initiate unspecified disciplinary measures if the religious order failed to discipline the two sisters to its satisfaction.

Hamer specifically noted that the two nuns, who operate a shelter for the homeless in Charleston, W. Va., had joined in signing the advertisement in The New York Times stating they did not consider the pope's stand against abortion as "the only legitimate Catholic position."

The religious order's governing council was not available for comment on Hamer's statement tonight. But Sister Elizabeth Bowyer, one of five members of the order's council, said last month that the two nuns' gravest proabortion act was their attendance at a "prochoice" rally in Washington March 9 "with no reference to the sanctity of life or the place of moral responsibility in decision-making."

Hamer made no reference to the Washington rally and kept his criticism of the two nuns' actions to their signing of the newspaper ad.

Last month, Bowyer said that her order, in accordance with Vatican demands, had asked the two nuns to "clarify their prochoice position" but that no proceedings for dismissal from the order were envisaged.

She said at the time that the order was not so much concerned with the sisters having signed two advertisements questioning the Vatican stand on abortion -- including a second one earlier this year -- as it was with their attendance at the Washington rally.

"The first advertisement was a call for dialogue and the second was in defense of dissent within the church," Bowyer said. "We do not see sufficient reason for dismissal in that situation."

"The main concern," she said, "is the position they took at the prochoice rally in Washington March 9.