TORONTO -- A Canadian indigenous group announced on Wednesday the "horrific and shocking discovery" of hundreds of unmarked graves at the site of a former residential school, just weeks after the discovery of other children's remains shook the country.

The Federation of Sovereign Indigenous Nations said in a statement that the number of newly found unmarked graves was "the most significantly substantial to date in Canada." The statement did not specify numbers.

The group said it would announce at a news conference on Thursday morning "the horrific and shocking discovery of hundreds of unmarked graves at the site of the former Marieval Indian Residential School" in Saskatchewan.

The discovery weeks ago of the remains of 215 indigenous children at the site of another residential school for indigenous children in Kamloops, British Columbia, forced Canadians to confront the legacy of an abusive and assimilationist system.

Between 1831 and 1996, Canada's residential school system forcibly separated about 150,000 indigenous children from their families. They were malnourished and physically and sexually abused in what the country's Truth and Reconciliation Commission called "cultural genocide" in 2015.

Survivors who spoke with Reuters recalled perpetual hunger and haunting loneliness, and schools run under the threat and frequent use of force. 

Canada's federal government apologized for the system in 2008. The Roman Catholic Church, which ran most of the schools, has not apologized. Earlier this month, Pope Francis said he was pained, a statement dismissed by survivors.