More than 1,000 children — and possibly many more — were molested by hundreds of Roman Catholic priests in six Pennsylvania dioceses and senior church officials took steps to cover it up, according to a landmark grand jury report.

The grand jury said it believed the "real number" of abused children might be "in the thousands" since some records were lost and victims were afraid to come forward.

The report said more than 300 clergy committed the abuse over decades, beginning in the mid-1950s.

Pennsylvania Attorney-General Josh Shapiro said the two-year probe found a systematic cover-up by senior church officials in Pennsylvania and at the Vatican.

"The cover-up was sophisticated.

"And all the while, shockingly, church leadership kept records of the abuse and the cover-up," he said.

"These documents, from the dioceses' own 'secret archives,' formed the backbone of this investigation."

The report faults Archbishop of Washington Cardinal Donald Wuerl (left) for his part in the concealment of abuse.

Significantly, the report faulted Cardinal Donald Wuerl, the former long-time bishop of Pittsburgh who now leads the Washington archdiocese, for what it said was his part in the concealment of clergy sexual abuse.

Cardinal Wuerl, one of the highest-profile cardinals in the United States, released a statement that said he had "acted with diligence, with concern for the victims and to prevent future acts of abuse".

Abuse described as horseplay

The grand jury scrutinised abuse allegations in dioceses that minister to more than half the state's 3.2 million Catholics.

Its report echoed the findings of many earlier church investigations around the country in its description of widespread sexual abuse by clergy and church officials' concealment of it.

Most of the victims were boys, but girls were abused, too, the report said.

The abuse ranged from groping and masturbation to anal, oral and vaginal rape.

"Church officials routinely and purposefully described the abuse as horseplay and wrestling and inappropriate conduct.

"It was none of those things. It was child sexual abuse, including rape," Mr Shapiro said.

The panel concluded a succession of Catholic bishops and other diocesan leaders tried to shield the church from bad publicity and financial liability by covering up abuse, failing to report accused clergy to police and discouraging victims from going to law enforcement.

Yet, the grand jury's work won't result in justice for the vast majority of those who say they were molested by priests as children.

While the probe yielded charges against two clergymen — including a priest who has since pleaded guilty, and another who allegedly forced his accuser to say confession after each sex assault — the other priests identified as perpetrators are either dead or will avoid arrest because their alleged crimes are too old to prosecute under state law.

"We are sick over all the crimes that will go unpunished and uncompensated," the grand jury said.

Clergy members' names sealed by court

McCarrick was ordered to remain in a house for a "life of prayer and penance" until a trial is held.

The document comes at a time of renewed scrutiny and fresh scandal at the highest levels of the US Catholic Church.

Pope Francis stripped 88-year-old Cardinal Theodore McCarrick of his title and ordered him to a lifetime of prayer and penance amid allegations that Cardinal McCarrick had sexually abused boys for years and also had sexual misconduct with adult seminarians.

Cardinal Wuerl has come under harsh criticism over his response to the McCarrick scandal, with some commentators questioning his claims of surprise and ignorance over allegations that Cardinal McCarrick molested and harassed young seminarians.

Cardinal Wuerl replaced him as Washington's archbishop after Cardinal McCarrick retired in 2006.

The Pennsylvania grand jury, convened by the state attorney general's office in 2016, heard from dozens of witnesses and reviewed more than a half million pages of internal documents from the Allentown, Erie, Greensburg, Harrisburg, Pittsburgh and Scranton dioceses.

Some current and former clergy named in the report went to court to prevent its release, arguing it violated their constitutional rights to reputation and due process of law.

The Supreme Court of Pennsylvania said the public had a right to see the report, but ruled the names of priests and others who objected to the findings would be blacked out pending a hearing on their claims in September.

The identities of those clergy members remain under court seal.

A couple of dioceses decided to strip the accused of their anonymity ahead of the report and released the names of clergy members who were accused of sexual misconduct.

The bishop of Pittsburgh's diocese said a few priests named in the report were still in ministry because the diocese determined allegations against them were unsubstantiated.