The Vatican has a 2,000-case backlog in processing clerical sex abuse cases, and criticism of the slow pace is justified, Pope Francis has said.

But he said that more staff were being added and insisted the Vatican is "on the right path".

Pope Francis was making his first comments about the criticism levelled at the Vatican's handling of sex-abuse cases by Marie Collins, an Irish abuse survivor who resigned from Pope Francis' sex abuse advisory commission in March.

Ms Collins quit because of what she said was the "unacceptable" level of resistance within the Vatican to implementing the group's proposals to better care for victims and protect children from priests who rape and molest them.

Speaking to reporters while flying home from a trip to Portugal, Pope Francis called Ms Collins "a great woman" and said she was "a bit right" to complain about the slow pace in processing cases.

"Marie Collins was right on that point. But we are on the right path, as there were 2,000 cases backlogged," he said.

Pope Francis did not respond to the other issues raised by Ms Collins, including the refusal of the Congregation for the Doctrine of the Faith, which handles abuse cases, to create a tribunal to judge bishops who covered up for paedophile priests.

Instead, he focused on explaining why cases can take so long to process.

To improve efficiency, he said he had added more staff to the congregation and that talks were under way about providing more regional help to bishops to ensure their cases are properly documented before they arrive at the Vatican for review.

He denied he had ever agreed to a request for clemency from a paedophile priest, saying once a sentence is confirmed on appeal by the congregation "it is finished".

Pope Francis has come under fire from some survivors and their advocates for his handling of the abuse crisis.

He won praise for having created the advisory commission and for promising "zero tolerance" for abuse.

But his advisory board has lost credibility following Ms Collins' departure and its failure to implement key recommendations that even Pope Francis had approved.

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