Tear gas was used to disperse a crowd of 400 protesters at the Dakota Access Pipeline late Sunday after clashes that authorities described as a "riot" prompted by "very aggressive" activists.
A law enforcement officer was hit on the head by a thrown rock during the confrontation, Morton County Sheriff's Office said in an update at 1 a.m. local time (2 a.m. ET).
Videos posted to Facebook by activists showed authorities spray a continuous stream of water over demonstrators in below-freezing temperatures but sheriff's spokesman Rob Keller told NBC News that no water cannon were deployed. He said the water was being sprayed from a fire truck to control blazes as they were being set by activists.
In an earlier release, the sheriff's office described the clashes as "an ongoing riot on the Backwater Bridge, north of a protest camp."
"Officers on the scene are describing protesters' actions as very aggressive," the release noted. "In order to keep protesters from crossing the bridge, law enforcement have utilized less-than-lethal means, including launching CS gas."
Walker River Paiute member Atsa E'sha Hoferer, who identifies as a "water protector," said he was hit with tear-gas and sprayed with water. Hoferer said demonstrators were lighting fires to provide warmth in the 25-degree weather.
"They're saying that we're causing multiple fires out here, but we're really only using them to stay warm," said Hoferer, 27. "I'm just a father with a phone that loves his water, that wants his water to be clean for his children and grandchildren."
The bridge had been closed since late October, when it was damaged in a fire after authorities evicted protesters from property owned by the pipeline developer.
The incident Sunday began at 6 p.m. (7 p.m. ET) after protesters removed a truck that had been there since Oct. 27, the statement said.
n an account posted on Facebook, Indigenous Environmental Network organizer Dallas Goldtooth said "water protectors" removed the vehicle to gain access to North Dakota Highway 1806.
"It was to open up the road so in the daylight the world can see the face of militarized law enforcement and state oppression," Goldtooth wrote. "Police in response are using a water cannon, tear gas and concussion grenades on the crowd!!"
By 8:30 p.m., the sheriff's office said, one person had been arrested.
The clash comes days after organizers held coordinated demonstrations across the country to protest a 1,170-mile oil pipeline that has generated the largest American Indian protest movement in modern history. Thousands of people have flocked to North Dakota, where the Standing Rock Sioux Tribe argues that the proposed pipeline could permanently contaminate its water source, the Missouri River, as well as desecrate sacred sites.
Energy Transfer Partners, which is behind the pipeline, has said that it has taken measures to prevent such leaks and that the pipeline is far safer than transporting oil with trucks or trains.