The US Justice Department has opened an investigation into child sex abuse by priests in Pennsylvania, two months after the publication of a report on decades of sexual abuse in the state.

The investigation is the first statewide probe by federal authorities of allegations of sex abuse and cover-up by the Catholic Church in the United States, according to groups representing abuse victims.

The Pennsylvania dioceses said they had received federal subpoenas following a state grand jury report that alleged over 300 Catholic priests in Pennsylvania had sexually abused children over 70 years.

The dioceses of Allentown, Erie, Harrisburg, Philadelphia, Pittsburgh and Scranton said they were cooperating with the investigation but declined further comment.

The Justice Department and US Attorney's Office in Philadelphia declined to comment.

An 884-page report made public in August by Pennsylvania Attorney General Josh Shapiro after a two-year investigation contained graphic examples of children being groomed and sexually abused by clergymen.

Mr Shapiro said at the time that it was largely based on documents from secret archives kept by the dioceses, including handwritten confessions by priests.

The report cited 301 priests, some of whom have died.

Tim Lennon, president of Survivors Network of those Abused by Priests (SNAP), said the investigation was long overdue.

Four statewide grand juries in Pennsylvania since 2003 identified around 500 alleged sexual predators among acting or former Catholic clergy, he said.

"In what other institution could you have 500 criminals and not be prosecuted?" asked Mr Lennon, who said he was abused by a Catholic priest when he was a child.

Mr Lennon said SNAP had asked the federal government three times since 2002 for a nationwide investigation of 15,000 active or retired Catholic clergy accused of being sexual predators.

In September, US Catholic bishops said they would set up a hotline for accusations of sexual abuse against bishops and other church leaders, or allegations of cover ups by such people.

The report is thought to be the most comprehensive to date into abuse in the US church.