In an interview with the Italian news agency AdnKronos, Pope Francis also said he listens to his critics but can't allow himself to get bogged down by every negative thing they say.

"Unfortunately corruption runs in cycles. It repeats itself. Someone comes along who cleans things up but it starts again until someone else comes along to put an end to this degeneration," he said.

In a shock move last month, Francis fired Cardinal Angelo Becciu, a former top Vatican official, accusing him of embezzlement and nepotism.

Becciu, who also has been caught up in a scandal involving the Vatican's purchase of a luxury building in London, has denied all wrongdoing.

Cecilia Marogna, a 39-year-old Italian who worked for Becciu, was released after more than two weeks in jail on Friday pending a judge's decision on an extradition request from the Vatican.

"I know I have to do it (fight corruption), I was called to do it, but it will be the Lord to decide if I did well or not. Sincerely, I am not very optimistic," Francis said, smiling.

He was referring to the mandate he received from cardinals who elected him in 2013 to clean up Vatican finances after a string of scandals during the pontificates of his two immediate predecessors, Pope Benedict and Pope John Paul II.

"There are cases of malfeasance, of betrayals, that wound those who believe in the Church. These people (corrupt churchmen) certainly don't behave like cloistered nuns."

Conservative Catholics have criticised the pontiff, saying he sends mixed signals on issues such the pastoral care of homosexuals and those who remarry civilly without Church annulments.

"Criticism can be constructive, and if it is I take it all on board because it can lead to self-examination ... but I can't let myself be dragged down by every non-positive thing written about the Pope," he said.

Australian Associated Press