Pope Francis has exhorted Catholic bishops worldwide to do what is needed to ensure children are protected from sexual abuse by clergy.

Key points: Letter asked bishops to take "all necessary measures" to protect "the lives of our children"

Letter asked bishops to take "all necessary measures" to protect "the lives of our children" Pope Francis decried the "pain of minors who were abused sexually by priests"

Pope Francis decried the "pain of minors who were abused sexually by priests" Church's reputation stained by allegations of child sex abuse at hands of clergy

The Vatican has released the text of a December 28 letter Pope Francis sent to bishops about injustices to children including slave labour, malnutrition, lack of education and sexual exploitation, including abuse by priests.

In the letter, Pope Francis decried "the sufferings, the experiences and pain of minors who were abused sexually by priests".

"It is a sin that shames us," the Pope wrote.

"Persons responsible for the protection of those children destroyed their dignity."

The Church's reputation has been stained in several countries during the last few decades as people have come forward to report that parish priests or other Catholic clergy raped or molested them as minors.

In Australia, the Catholic Church will be fronting the Royal Commission into Institutional Responses to Child Sexual Abuse in coming months to explain improvements to its policies.

Worldwide, the allegations showed that local bishops sometimes knew about and covered up child sex abuse involving problem priests and triggered multi-million-dollar lawsuits, as well as several criminal prosecutions.

Pope calls for 'zero tolerance' on child sex abuse

Expressing the Church's regret, and begging forgiveness, the Pope denounced the "sin of what happened, the sin of failing to help, the sin of covering up and denial, the sin of the abuse of power".

Pope Francis also asked bishops for "complete commitment to ensuring that these atrocities will no longer take place in our midst".

"Let us find the courage needed to take all necessary measures and to protect in every way the lives of our children, so that such crimes may never be repeated," the Pope said.

"In this area, let us adhere, clearly and faithfully, to 'zero tolerance'."

The pontiff himself has received mixed reviews on how the Vatican handles sex abuse.

Pope Francis has laid out procedures to oust bishops for negligence, if they mishandle investigations into alleged abuse.

But he dismayed advocates for abuse survivors by appointing a Chilean bishop accused of covering up for a notorious paedophile.

The Vatican also took no immediate action after deaf students from Italy, in a 2014 letter to the Pope, said a priest sexually abused them for years in Italy and now was working at a school in Francis' native Argentina.

The priest was arrested last year and charged with raping deaf students at a school in Argentina.

AP