Pope Francis became the first pontiff to openly admit to and condemn a 'culture of abuse and cover-up' in the Catholic Church.

Francis promised Chilean Catholics that 'never again' would the Church ignore them or the cover-up of abuse in their country, where a widespread scandal has devastated its credibility. 

The pope issued the comments in a letter to all Chilean Catholics on Thursday as the Vatican announced that Francis was sending his two top sexual abuse investigators back to the country to gather more information about the crisis there.

The Vatican's most experienced sexual abuse investigator, Archbishop Charles Scicluna of Malta, and Father Jordi Bertomeu, a Spaniard, had visited Chile earlier this year.

In the letter released by Chilean bishops, Francis also praised the victims of sexual abuse in the country for persevering in bringing the truth to light despite attempts by Church officials to discredit them.

'The "never again" to a culture of abuse, and the system of cover-up that allowed it to perpetuate, calls on all of us to work towards a culture of carefulness in our relationships,' he said in the eight-page letter.

He described the Chilean scandal as a 'painful open wound'.

Hours before the letter was released in Chile, the Vatican said Scicluna and Bertomeu would concentrate on the diocese of Osorno in southern Chile, seat of a bishop who has been most caught up in the scandal.

A Vatican statement said the purpose of the trip, due to start in the next few days, was to 'move forward in the process of reparation, and healing for victims of abuse'.

The two prepared a 2,300-page report for the pope after speaking to victims, witnesses and other Church members earlier this year.

On May 18, all of Chile's 34 bishops offered to resign en masse after attending a crisis meeting with the pope in the Vatican about the cover-up of sexual abuse in the south American nation.

Francis has not yet said which resignations he will accept, if any. In his letter, the pope said the renewal of the Church hierarchy on its own would not bring the transformation needed in Chile, calling for unity in a time of crisis and a deepening of faith.

The scandal revolves around Father Fernando Karadima, who was found guilty in a Vatican investigation in 2011 of abusing boys in Santiago in the 1970s and 1980s. Now 87 and living in a nursing home in Chile, he has always denied any wrongdoing.

Victims accused Bishop Juan Barros of Osorno of having witnessed the abuse but doing nothing to stop it. Barros, who was one of those who offered to stand down, has denied the allegations.

During a visit to Chile in January, Francis staunchly defended Barros, denouncing accusations against him as 'slander'.

But days after returning to Rome, the pope, citing new information, dispatched Scicluna and Bertomeu to Chile.

Some of their findings were included in a damning 10-page document that was presented to the bishops when they came to Rome.

In April, the pope hosted three non-clerical victims who said they were abused by Karadima, and this weekend he will be meeting with priests who said they were abused by Karadima when they were young.

Additional reporting by Reuters.