Spain's Catholic Church sought to give more credence to an investigation into the alleged sexual abuse of minors, saying on Tuesday a law firm would coordinate and audit the effort, in a move a victims' group has already dismissed as a "smoke screen".

The allegations are only gaining traction now in Spain, years after similar scandals rocked the Church in other countries such as the United States, Ireland and France. 

Spanish prosecutors said last week they were investigating 68 cases, but El Pais newspaper reported two months ago it had found 1,200 cases reported between 1943 and 2018.

"We are deeply saddened by the abuses that have happened at the institution. The Bishops' Conference wants to take a step in its duty of bringing transparency, help and reparation to the victims, and collaboration with the authorities," Cardinal Juan Jose Omella, head of the Spanish Church, told a news conference.

Eighteen staff of Cremades&Calvo-Sotelo law firm will, over the next 12 months, conduct and audit the investigation free of charge, complementing the work of commissions set up at diocese level and other probes begun or planned by Spanish authorities.

The firm's head, Javier Cremades, who disclosed he was a member of the conservative Catholic group Opus Dei, said the Church had to fully investigate the alleged abuses, and that his firm was providing a service both to victims and society. 

"I'm here as a lawyer, not a faithful," he said, promising to take the best of other countries' investigations, such as those in France or Germany.

But Fernando Garcia-Salmones, a spokesman for victim's association Stolen Childhood, was concerned about the strong ties between the Catholic community and the law firm, calling the move "another smoke screen" to keep a lid on abuse cases.

"They want their faithful to protect them," he told reporters on Monday. "Had the Church had real intentions of doing a thorough investigation, it would have done it by now."