(CNN) - Catholic bishops "will be held accountable" for failing to protect children from sexual abuse, Pope Francis said Monday, his strongest acknowledgment yet of what abuse victims have been saying for decades: that the cover-ups have often been as bad as the crimes.

But without strong action to back up those words, such groups are likely to view Francis' comments as little more than lip service. Vatican officials have so far been reluctant to take action against bishops accused of concealing abuse.

In a homily given during a private Mass with six victims of church sexual abuse, Francis apologized for the abuse and asked for forgiveness.

"I beg your forgiveness, too, for the sins of omission on the part of Church leaders who did not respond adequately to reports of abuse made by family members, as well as by abuse victims themselves," Francis said in the homily, according to a text of the statement provided by the Vatican.