The remains of 215 children have been discovered on the grounds of a former boarding school set up more than a century ago to assimilate Canada's indigenous peoples, according to a local tribe.

Lorelei Williams puts one of 215 pairs of children's shoes on the steps of the Vancouver Art Gallery on Friday. Photo: VCG

A specialist used ground-penetrating radar to confirm the remains of the students who attended the school near Kamloops, British Columbia, the Tk'emlups te Secwepemc tribe said in a statement late Thursday. "Some were as young as three years old," said chief Rosanne Casimir, calling it "an unthinkable loss that was spoken about but never documented" by school administrators.

Its preliminary findings are expected to be released in a report in June, she said.

In the meantime, the tribe is working with the coroner and museums to try to shed further light on the horrific discovery and find any records of these deaths.

It is also reaching out to the students' home communities across British Columbia and beyond. The "distressing" discovery of the remains "breaks my heart," Prime Minister Justin Trudeau said in a Twitter message.

"It is a painful reminder of that dark and shameful chapter of our country's history," he said.

His comments were echoed by Crown-Indigenous Relations Minister Carolyn Bennett who also offered government support to the families and indigenous communities for their "healing as we honor loved ones lost."

The Kamloops Indian Residential School was the largest of 139 boarding schools set up in the late 19th century, with up to 500 students registered and attending at any one time.