Indigenous leaders are demanding answers about what happened in a video that appears to show a Thunder Bay, Ont. police officer hitting a First Nations teenager who is strapped to a stretcher, as the local force says it's investigating.

"We are outraged by the actions of the officer depicted in this video. We do not know all of the details that led to this incident, but there is simply no justification for such violent and callous treatment of a youth in such a defenceless position," Alvin Fiddler, grand chief of the Nishnawbe Aski Nation (NAN) which represents 49 First Nations in Northern Ontario, said in a statement on Sunday.

Fiddler called for an independent investigation into the matter with its results made public.

NAN said the person on the stretcher is a 17-year-old from Nibinamik First Nation, a remote community about 500 kilometres north of Thunder Bay, who is attending school at the Matawa Learning Centre.

Thunder Bay police spokesperson Chris Adams said the force is aware of the video and is investigating.

He declined to provide further details about the investigation. NAN officials said it remains unclear whether the probe will be independent of the local police service.

'You do not spit on me,' officer is heard yelling

The video that is circulating online appears to show a Thunder Bay police officer and a paramedic standing over the teen who is strapped to a stretcher near an ambulance.

The officer and paramedic appear to be holding down the youth's arms that broke free.

In the 21-second video, obtained by CBC News, a female officer can be heard yelling, "That's enough," before appearing to hit the teen in the face.

Afterward, the officer yells: "Do not spit on me, you do not spit on me," while appearing to forcefully push down the person in the stretcher.

Prior to that, a voice can be heard saying, "you're going to the hospital."

Nishnawbe Aski Nation officials said it's not clear why the youth required medical attention.