BERLIN (Reuters) - Pope Francis - in comments in the foreword of a new book - has branded sexual abuse of children by Roman Catholic priests a "monstrosity" and pledged action against perpetrators and bishops who protected them.

The book titled "Father, I Forgive You: Abused But Not Broken" was written by Swiss man Daniel Pittet, 58, who was first raped by a priest when he was eight years old.

Francis, whose repeated promises of zero tolerance have been criticized by victims who say the Vatican needs to do much more, called sexual abuse "an absolute monstrosity, a terrible sin that contradicts everything that the Church teaches".

The foreword was published on Wednesday by the mass circulation German daily Bild.

Francis said the fate of abused children weighed on his soul, especially those who had taken their own lives.

"We will counter those priests who betrayed their calling with the most strenuous measures. This also applies to the bishops and cardinals who protected these priests – as happened repeatedly in the past," the pope wrote.

Church sexual abuse broke into the open in the United States with reports of cases in Louisiana in 1984 and exploded in 2002, when journalists in Boston found that bishops had systematically moved abusers to new posts instead of defrocking them.

Thousands of cases have come to light around the world as investigations have encouraged long-silent victims to go public, shattering the Church's reputation in places such as Ireland, and more than $2 billion has been paid in compensation.

DISSENT

Francis' efforts against sexual abuse since his election in 2013 have sputtered.

Critics say he has not done enough to hold to account those bishops who mishandled cases of abuse or covered it up, and a Vatican commission formed in 2014 to advise him on rooting it out has been hit by internal dissent.

Peter Saunders, an English victim of clergy abuse, took a leave of absence last year in protest over a lack of progress.

Marie Collins from Ireland, also a victim of priestly abuse when she was a child, quit in frustration in March, citing a "shameful" lack of cooperation within the Vatican.

Francis praised Pittet's courage in telling his story, saying he was deeply moved by his ability to forgive his abuser 44 years after he was first molested. The Church has defrocked the abuser.

Pittet, who endured four years of rapes, abuse and exposure to pornography, wrote that his act of forgiveness had nothing to do with human justice or denial.

"Forgiveness does not heal the wounds or wipe away the misery ... forgiving him has allowed me to burst the chains that bound me to him and prevented me from living," Pittet wrote in the book, according to excerpts released by German publisher Herder.

Pittet, who lives in the Swiss city of Fribourg, works as a librarian and has six children.