Pope Francis received a victim's letter in 2015 that detailed sexual abuse by a priest and a cover-up by Chilean church authorities, according to the Associated Press, in a report that contradicted the Pope's recent insistence that victims 'haven't come forward'.

The author of the eight-page letter and members of Pope Francis' own sex abuse commission told the AP about the letter, which the news agency said challenges the pope's self-described policy of 'zero tolerance' for sex abuse and cover ups and calls into question his stated empathy with victims, representing the biggest crisis of his five year papacy.

Francis's trip to South America was overshadowed by protests over his vigorous defence of Bishop Juan Barros, who is accused by victims of covering up the abuse by Fr Fernando Karadima.

The pope dismissed accusations against Barros as 'slander'.

Questioned by journalists on the plane back to Rome, the pope said: 'You, in all good will, tell me that there are victims, but I haven't seen any, because they haven't come forward.'

After Pope Francis's comments backing the Chilean hierarchy caused such an outcry in Chile, the Vatican last week sent in its most respected sex-crimes investigator to take testimony from Cruz and others about Bishop Barros.

Now, members of the Pope's Commission for the Protection of Minors have told AP that in April 2015, they sent a delegation to Rome specifically to hand-deliver a letter to the Pope about Bishop Barros.

The letter, from Juan Carlos Cruz, detailed in graphic detail the abuse he says he suffered at Fr Karadima's hands, which he said Barros and others witnessed and ignored.

Four members of the commission reportedly met with Pope Francis' top abuse adviser, Cardinal Sean O'Malley, explaining their opposition to the pope's recent appointment of Barros as a bishop in southern Chile, and gave him the letter to deliver to Francis.

'When we gave him [O'Malley] the letter for the pope, he assured us he would give it to the pope and speak of the concerns,' the then-commission member Marie Collins told the AP. 'And at a later date, he assured us that that had been done.'

Cruz, who now lives in Philadelphia, said that he heard the same later that year. He told AP: 'Cardinal O'Malley called me after the Pope's visit here in Philadelphia and he told me, among other things, that he had given the letter to the Pope — in his hands.'

In the letter, Cruz wrote in Pope Francis's native Spanish. 'Holy Father, I write you this letter because I'm tired of fighting, of crying and suffering,' he wrote. 'Our story is well known and there's no need to repeat it, except to tell you of the horror of having lived this abuse and how I wanted to kill myself.'

Neither the Vatican nor O'Malley responded to a number of requests for comment from the AP.

What has become known as the Barros affair first erupted in January 2015 when Francis appointed him bishop of Osorno, Chile, despite objections from the leadership of Chile's bishops' conference and many local priests and laity. They accepted the testimony against Fr Karadima, a prominent Chilean cleric who was sanctioned by the Vatican in 2011 for abusing minors. Bishop Barros was a Karadima 'protege', according to AP, and the agency said that according to Cruz and other victims, he witnessed the abuse and did nothing.

In the letter to the pope, Cruz begs for the pope to listen to him and make good on his commitment to 'zero tolerance'.

He wrote: 'Holy Father, it's bad enough that we suffered such tremendous pain and anguish from the sexual and psychological abuse, but the terrible mistreatment we received from our pastors is almost worse.'

Bishop Barros has repeatedly denied witnessing any abuse or covering it up. He told AP recently: 'I never knew anything about, nor ever imagined, the serious abuses which that priest committed against the victims. I have never approved of nor participated in such serious, dishonest acts, and I have never been convicted by any tribunal of such things.'