Tension flared anew on the Dakota Access pipeline as protesters tried to push past a long-blocked bridge on a state highway, only to be turned back by a line of law enforcement using water cannon and what appeared to be tear gas.

Sunday's skirmishes began around 6 pm after protesters removed a burned-out truck on what's known as the Backwater Bridge, not far from the encampment where they've been for weeks as they demonstrate against the pipeline. The Morton County Sheriff's Department estimated 400 protesters sought to cross the bridge on state Highway 1806.

A live stream early on Monday showed a continued standoff, with large lights illuminating smoke wafting across the scene.

The sheriff's department said in a statement around 1 am on Monday that approximately 100 to 200 protesters were on the bridge or in the vicinity. It said law enforcement officers "had rocks thrown at them, burning logs and rocks shot from slingshots", and that one officer had been hit on the head by a thrown rock.

At least one person was arrested. Protesters said a gym in Cannon Ball was opened to aid demonstrators who were soaked on a night the temperature dipped into single digits or were hit with tear gas.

Rema Loeb told The Associated Press he was forced to retreat from the bridge because he feared being doused with water on the freezing night. Others, he said, needed medical treatment after being hit with tear gas.

"It's been just horrible," said the 83-year-old Loeb, who traveled from Massachusetts about two weeks ago to join the protests.

Concerns

The 1,900-kilometer, four-state pipeline is being built to carry oil from western North Dakota to a shipping point in Illinois. But construction of the $3.8-billion pipeline has been protested for months by the Standing Rock Sioux, whose reservation lies near the pipeline route, and the tribe's allies, who fear a leak could contaminate their drinking water. They also worry that construction could threaten sacred sites.

On Friday, Kelcy Warren, chief executive of Energy Transfer Partners, said the company is unwilling to reroute the pipeline.