The UN has slammed the Vatican for "allowing" the rape and sexual abuse of thousands of children for decades.

In a damming report by the UN watchdog for children's rights, the Holy See has been accused of not taking adequate action to fight paedophile priests and bring them to justice.

"The Holy See has not acknowledged the extent of the crimes committed, has not taken the necessary measures to address cases of child sexual abuse and to protect children, and has adopted policies and practices which have led to the continuation of the abuse by, and the impunity of, the perpetrators," the UN Committee on the Rights of the Child (CRC) said.

The UN body said it was deeply concerned "about child sexual abuse committed by members of the Catholic churches who operate under the authority of the Holy See, with clerics having been involved in the sexual abuse of tens of thousands of children worldwide".

The CRC particularly criticised the Vatican practice to transfer "well-known child sexual abusers" to other parish or other countries to cover up their crimes.

"The practice of offenders' mobility, which has allowed many priests to remain in contact with children and to continue to abuse them, still places children in many countries at high risk of sexual abuse," the report read.

In its recommendations, the committee urged the Vatican to open its files on sexual offenders and the members of the clergy who concealed their crimes.

It also called for the establishment of clear rules for the mandatory reporting of sex abuses to police.

"Due to a code of silence imposed on all members of the clergy under penalty of excommunication, cases of child sexual abuse have hardly ever been reported to the law enforcement authorities in the countries where such crimes occurred," the CRC wrote.

"On the contrary, cases of nuns and priests ostracized, demoted and fired for not having respected the obligation of silence have been reported to the Committee".

Pope Francis set up a commission to fight sex abuse in the Church in December.

The UN pressed the new Vatican body to conduct an independent investigation into all historic cases of priestly abuse in the Catholic Church.

The report came after the committee questioned Vatican officials on the Holy See's implementation of the UN Convention on the Rights of the Child.

The committee's recommendations are non-binding, however the UN has asked the Vatican to implement them and report back by 2017.