Alberta's Child and Youth Advocate says he doesn't believe there is any way to guarantee foster children will never be abused.

John Mould, Alberta's Child and Youth Advocate, told CBC News allegations that a 51-year-old Calgary man allegedly offered money to three boys in his care in exchange for sexual acts are disappointing.

However, Mould said cases like this one are one of the realities of child welfare systems.

"You know, it's one of those things. One never wants it to happen but … it does happen," he said, adding that his office will work "to learn as much as possible about the circumstances of what happened here."

Children and Youth Services Minister Yvonne Fritz has called for an internal review of the case.

She said she will act quickly to implement any recommendations that might come out of the report.

But Mould said there is no way to guarantee that children are 100 per cent safe.

"There's no fool-proof screening mechanism that I'm aware of," he said.

"I think that lots and lots of effort is put into doing the best screening that the service system knows how to do … but I just don't think that there is an answer that would reassure people that if we did this the problem would disappear."

While the case has the province's Liberal party questioning the competency of the Department of Children and Youth Services, Fritz said incidents of abuse within the foster care system are very unusual.