(NBC News/Tim Stelloh, Molly Roecker and Chiara Sottile) — Clashes between protesters and authorities over the Dakota Access Pipeline escalated Sunday night after an estimated 400 people tried to breach a law enforcement barrier, North Dakota law enforcement officials said.

The Morton County Sheriff's Office described the clash as a "riot" prompted by "very aggressive" activists, while protesters said authorities used tear gas, rubber bullets and water cannon in below-freezing temperatures.

NBC News couldn't independently confirm the use of rubber bullets, and a 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 are being set by activists.

Atsa E'sha Hoferer of the Walker River Paiute Tribe, who called himself 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."

In a statement, the sheriff's office acknowledged that tear gas and other "less than lethal means" were being used after protesters "engaged in organized tactical movement and attempted to flank and attack" a law enforcement line near a bridge.

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.

In 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!!"

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