Pope Francis has apologised for remarks he made last week in Chile defending a bishop accused of covering up sexual abuse.
He said he realised his words hurt many, but repeated his belief that Chilean Bishop Juan Barros was innocent.
Francis was speaking to journalists on board a plane flying back to Rome.
On Thursday, he had said that victims who had accused Bishop Barros were committing slander.
The Pope was openly criticised by Cardinal Sean O’Malley of Boston, who said he left victims of sexual abuse committed by priests feeling abandoned.
“I apologise to them if I hurt them without realising it, but it was a wound that I inflicted without meaning to,” said the Pope on Monday, quoted by Reuters news agency. “It pains me very much.”
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Bishop Barros has not been accused of abuse, but of being present when another priest, Fernando Karadima, molested young boys.
A rare apology
Analysis by James Reynolds, BBC Rome correspondent
It is unusual for a Pope to apologise for his own words. But he clearly felt that he had to make up for his abrupt dismissal of allegations made by victims of clerical sexual abuse in Chile.
The victims had accused a Chilean Bishop, Juan Barros, of covering up abuse committed by a fellow priest.
During his trip to Chile, the Pope described these accusations as a slander, for which there was not a shred of evidence.
The Pope has now apologised for his choice of words. He said that he understood that by essentially daring victims to bring him proof, it came off as a slap in the face.
However, the Pope continued to insist that an investigation had shown that there was no evidence to support the charges levelled against Bishop Barros.
What is the controversy about?
In 2010, Father Karadima was publicly accused of molesting several teenaged boys in the capital, Santiago, starting in the 1980s.
After the Vatican found him guilty, he was sentenced to a lifetime of “penance and prayer”.
He never faced criminal prosecution in Chile as too much time had passed, but the judge who heard victims’ testimony in a year-long investigation described them as “truthful and reliable”.
The Pope has been criticised for appointing Bishop Barros as bishop of Osorno, in south-central Chile, in 2015. The ordination ceremony had to be cut short over protests in the cathedral.
How did this affect his latest trip?
The Pope was greeted with protests in Santiago, with demonstrators insisting the bishop should not hold his role.
Francis also met victims of sexual abuse by priests in the country. He cried with them and said he felt “pain and shame” over the scandal.
Some of the victims said saying sorry was no longer enough, and they want him to take action.