In a letter to Catholics worldwide following damning new revelations of misconduct in the US, Francis sought to project a get-tough response to the perpetrators and a compassionate shoulder for victims ahead of a fraught trip to Ireland this weekend.

Francis begged forgiveness for the pain suffered by victims and said lay Catholics must be included in the effort to root out abuse and cover-up. He blasted the clerical culture that has been blamed for the crisis, with church leaders more concerned about their own reputations than the safety of children.

"We showed no care for the little ones," Francis wrote. "We abandoned them."

But Francis alone can sanction bishops and he offered no hint that he would change the Vatican's longstanding practice of giving religious superiors a pass when they botch abuse cases or are negligent in protecting their flocks.

He said he was aware of the "effort and work being carried out in various parts of the world" to ensure children are protected and hold accountable those who commit abuse and cover it up.

But he made no reference to what the Vatican plans to do, saying only: "We have delayed in applying these actions and sanctions that are so necessary, yet I am confident that they will help to guarantee a greater culture of care in the present and future."

The letter was viewed by abuse survivors as little more than recycled rhetoric that fails to acknowledge the Vatican's own role in turning a blind eye to predatory priests and fomenting a culture of secrecy that has allowed crimes to go unpunished for decades.

"That culture was overseen by #Vatican & codified into its laws," tweeted Colm O'Gorman, a prominent Irish abuse victim who is organising a solidarity demonstration of survivors in Dublin during Francis' visit. "He needs to name & own that."

Marie Collins, another prominent Irish survivor who resigned in frustration from the pope's sex-abuse advisory commission, said statements about how terrible abuse is and how bishops must be held accountable are meaningless.

"Tell us instead what you are doing to hold them accountable," she tweeted. "That is what we want to hear. 'Working on it' is not an acceptable explanation for decades of 'delay."'

Priestly sex abuse was always expected to dominate the pope's trip to Ireland, a once staunchly Roman Catholic country where the church's credibility has been devastated by years of revelations that priests raped and molested children with impunity and their superiors covered it up.

Australian Associated Press