A Canadian indigenous group on Wednesday announced the discovery of hundreds of unmarked graves at the site of another former residential school for indigenous children.

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 further details about "the horrific and shocking discovery of hundreds of unmarked graves at the site of the former Marieval Indian Residential School" in Saskatchewan at a news conference on Thursday.

Last month the remains of 215 indigenous children, some as young as three years old, were found at the site of a residential school for indigenous children in Kamloops, British Columbia.

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.

In 2008, the Canadian government formally apologized for the system.

(Cover: Tributes outside a former indigenous school in Kamloops, Canada, where 215 graves were found in May. /AFP)

(With input from Reuters)