UN rights experts on Friday urged Ottawa and the Vatican to hold swift and thorough investigations into the discovery of unmarked graves at an indigenous religious school in western Canada.

"We urge the authorities to conduct full-fledged investigations into the circumstances and responsibilities surrounding these deaths, including forensic examinations of the remains found, and to proceed to the identification and registration of the missing children," the nine experts said.

United Nations experts do not speak for the global body but are mandated by the UN to report their findings to it.

The experts further called on Ottawa to undertake similar investigations in all other indigenous residential schools in Canada, saying victims had a right to know the full extent of any violations that occurred.

"The judiciary should conduct criminal investigations into all suspicious deaths and allegations of torture and sexual violence against children hosted in residential schools -- and prosecute and sanction the perpetrators and concealers who may still be alive," they added.

The experts included the UN special rapporteurs on the rights of indigenous people, the sexual exploitation of children, degrading treatment, and the chair of the working group on enforced disappearances.

They claimed that large-scale human rights violations had been perpetrated against indigenous children.

"It is inconceivable that Canada and the Holy See would leave such heinous crimes unaccounted for and without full redress," they said.

The Kamloops Indian Residential School in British Columbia was operated by the Catholic church on behalf of Ottawa from 1890 to 1969.

It was one of of 139 boarding schools set up a century ago to forcibly assimilate Canada's indigenous peoples.

The unmarked graves of 215 children were discovered at Kamloops last week, using ground-penetrating radar.

The UN experts urged the Roman Catholic Church to provide judicial authorities with full access to the archives of residential schools, "conduct prompt and thorough internal and judicial investigations into these allegations, and to publicly disclose the result of those investigations".