An Indigenous group in western Canada said Thursday it had discovered evidence of possible unmarked graves and a fragment of a child's jawbone on the grounds of a former residential school.

In Lebret, Saskatchewan, ground-penetrating radar has uncovered nearly "2,000 areas of interest" that need to be thoroughly investigated, the Star Blanket Cree community said.

Over the past year and a half, more than 1,300 children's graves have been found near government-run compulsory boarding schools where Indigenous children were forcibly assimilated.

The grisly discoveries have sent shock waves through Canada and raised national awareness of the dark past of how Indigenous people were treated.

Lebret is 80 kilometers (50 miles) northeast of Regina, Saskatchewan's capital.

A precise figure for the number of graves is not yet possible because not every "area" necessarily contains human remains, said Sheldon Poitras, who led the search.

Poitras and his team also discovered a fragment of a child's jawbone dating to about 125 years ago, the "physical proof of an unmarked grave," Poitras noted.

"Our hearts are heavy today," said Michael Starr, the community leader. "It was unthinkable."

The search areas were selected near the Catholic-run residential school, which was open until 1998, on the advice of former students.

On Thursday, Prime Minister Justin Trudeau called the announcement "difficult" and admitted that "the work is just beginning," promising the government's help throughout the investigation.

"The finding of human remains of a very young child at the site of Lebret Residential School is not only a tragic reminder of Canada's painful history and of the heinous acts that were committed in residential schools, it's further proof of that," said Marc Miller, minister of Crown Indigenous relations.

Between the late 19th century and the 1990s, some 150,000 Indigenous children were forcibly enrolled in 139 residential schools across the country, where they were cut off from their families, language and culture.

Thousands never returned.

A national commission of inquiry in 2015 called the system "cultural genocide."

Pope Francis travelled to Canada last year and apologized to Indigenous leaders for what he called the "evil" committed at the country's Catholic-run residential schools.