A protest over a four-state, $3.8 billion oil pipeline turned violent over the weekend in North Dakota.
Tribal officials say construction crews destroyed American Indian burial and cultural sites on private land.
Donnell Preskey, spokeswoman for the Morton County Sheriff's Office, said four private security guards and two guard dogs were injured when several hundred protesters confronted them just outside the Standing Rock Sioux reservation.
Tribe spokesman Steve Sitting Bear said six protesters were bitten by security dogs and at least 30 people were pepper-sprayed.
Preskey said authorities were not on the scene when the incident happened and the crowd dispersed when they arrived.
"Any suggestion that today's event was a peaceful protest, is false," Morton County Sheriff Kyle Kirchmeier told UPI. "This was more like a riot than a protest. Individuals crossed onto private property and accosted private security officers with wooden posts and flag poles. The aggression and violence displayed here today is unlawful and should not be repeated."
Although no arrests were made, the Bureau of Criminal Investigation is looking into the incident.
Protestors marched from property that belongs to the U.S. Army Corps of Engineers to private property on the west side of Highway 1806, according to UPI.
Native Americans began protesting the 1,100-mile pipeline on Aug. 10 when officials broke ground on the project near a reservation in southern North Dakota.
Tribe members said they fear the project will destroy sacred sites and impact drinking water.
The Associated Press contributed to this report.