VATICAN CITY, March 20 (AP) -- The Vatican on Tuesday denied a report in the National Catholic Reporter that says sexual abuse of nuns by priests, especially in AIDS-ravaged Africa, is a serious problem.

The article is based on five reports by senior members of women's religious orders and a priest going back to 1994. The National Catholic Reporter said the reports have been discussed at top Vatican levels.

Vatican spokesman Joaquin Navarro-Valls acknowledged there were isolated cases of priests sexually abusing nuns, but said the problem is ``restricted to a limited geographic area.'' The National Catholic Reporter story said the problem was most widespread in sub-Saharan Africa and Asia.

While the United States was mentioned, it was unclear from the story just how widespread -- or isolated -- the problem was among American priests and nuns.

Navarro-Valls said the Vatican was working with the leaders of religious orders and he stressed the ``heroic'' work of many priests and nuns.

According to the National Catholic Reporter, priests afraid of contracting AIDS sometimes turn to young nuns, who are seen as ``safe'' sexual partners.

``In a few extreme instances, according to the documentation, priests have impregnated nuns, then encouraged them to have abortions,'' the newspaper said.

In one case reported by Sister Maura O'Donohue, a physician and member of the Medical Missionary of Mary, a priest impregnated a nun, then arranged for an abortion. She died during the abortion and he officiated at her funeral Mass.

The reports cited by the National Catholic Reporter also say that nuns who become pregnant are often forced to leave their orders, while the priests involved are allowed to continue their ministries.

The Vatican statement was issued Tuesday after the Italian media rehashed the article. The Vatican had refused comment to the National Catholic Reporter.

The National Catholic Reporter said there are no comprehensive statistics on the sexual abuse of nuns, but the ``frequency and consistency of the reports ... point to a problem that needs to be addressed.''

The reports cited by the National Catholic Reporter link the problem in Africa and other developing regions to cultural attitudes toward celibacy, as well as to the fear of AIDS and to traditions of female subservience.

The Rev. Bernardo Cervellera, director of Fides, the news agency of the Vatican's missionary arm, said celibacy has always been a struggle for some priests, but he was surprised at the accounts of sexual abuse.

``I was a missionary for 25 years and I never encountered such a problem,'' he said. ``Instead, I found priests and nuns who gave themselves wholly to people with leprosy, with AIDS ... priests and nuns who live their love for Christ.''