CANNES Lily Gladstone, who grew up on the Blackfeet Indian Reservation and stars in Martin Scorsese's study of white society's treachery, said the director was a powerful ally in telling the world what communities like hers had always known.

In the film "Killers of the Flower Moon", which premiered at this year's Cannes Film Festival, Gladstone plays Mollie Burkhart, a member of the Osage Nation whose family members die under suspicious conditions in 1920s Oklahoma.

Because of his global reputation, Scorsese was uniquely placed to dispel the myths that have prevailed, she said.

"Who else is going to challenge people to challenge their own complicity in white supremacy ... except for this man here? Other artists are doing that work - people listen to what this one says," she said. "We need these allies."

Referring to deceptions that have prevailed, Gladstone asked: "Why the hell does the world not know about these things? Our communities always have."

In the film, Gladstone marries her white driver Ernest Burkhart (Leonardo DiCaprio), whose uncle is the "King of the Osage Hills" William Hale (Robert De Niro).

Hale presents himself as a friend of the oil-wealthy Osage, while instigating their murders to cash in on the deaths.

The director, who filmed entirely on location in Oklahoma, said the more he learnt about the Osage, the more he wanted to put into the nearly four-hour film.

"I wanted to know everything I could about the Osage, and it's overwhelming," he said.

Chief Standing Bear, principal chief of the Osage Nation, said Scorsese had restored trust.

"My people suffered greatly. To this very day, those effects go with us. But I can say on behalf of the Osage, Scorsese and his team have restored trust and we know that trust will not be betrayed," Chief Standing Bear told journalists in Cannes.

(Reporting by Miranda Murray; editing by Barbara Lewis)