Almost 700 clergymen in the US state of Illinois have been accused of child sexual assault, a far greater number than the Catholic Church had previously disclosed, the state's top prosecutor has said.

Illinois Attorney General Lisa Madigan said the church's revelations that 185 clergy members were credibly accused of sexual abuse fell short of the number her office has uncovered.

The preliminary results of an investigation that began in August found more than 500 additional priests and clergy members with sexual abuse allegations in the state's six dioceses - a total of at least 685 accused.

In a scathing statement, the attorney general's office criticised the church's handling of the abuse allegations, saying investigations were lacking, and in many cases law enforcement and child welfare authorities were not notified.

"The preliminary stages of this investigation have already demonstrated that the Catholic Church cannot police itself," Ms Madigan said.

She added that the church had failed to provide "a complete and accurate accounting of all sexually inappropriate behavior involving priests in Illinois."

The Illinois investigation was prompted by a sweeping grand jury report in August that revealed credible allegations against more than 300 suspected predator priests and identified over 1,000 victims of child sex abuse covered up for decades by the Catholic Church in the state of Pennsylvania.

Lisa Madigan said the church cannot police itself

In October, federal authorities for the first time opened an investigation into clergy abuse.

Dioceses in the state reported receiving federal grand jury subpoenas to produce documents.

The Archdiocese of Chicago, the largest of the Illinois dioceses, countered Ms Madigan's report by insisting that all abuse claims are investigated and reported to authorities.

"Since 2006, we have published the names of diocesan priests with substantiated allegations of abuse, and in 2014 we released more than 20,000 documents from these priests' files," the archdiocese said in a statement.

But Ms Madigan's office said allegations of abuse have often not been adequately investigated, if they are scrutinized at all.

Among the reasons for the lack of action were that the accused was deceased or had already resigned.

"This report is both shocking and exactly what we expected," Zach Hiner, executive director of the Survivors Network of those Abused by Priests (SNAP) said.

"We've known for a long time that church officials have been ignoring and minimizing allegations of abuse and this report is just yet another proof point that it is a systemic issue, not a highly localised one."

Since the state investigation opened, the dioceses have added another 45 clergy members to their official lists of those credibly accused of committing child sexual abuse, according to Ms Madigan's office.

The attorney general anticipated additional names will be disclosed as her investigation continues.

"Allegations of sexual abuse of minors, even if they stem from conduct that occurred many years ago, cannot be treated as internal personnel matters," Ms Madigan said.