Police and protesters have clashed near the site of the Dakota Access pipeline, with at least one person arrested.
Fresh tensions flared 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 officers using water cannon and what appeared to be tear gas.
Sunday's skirmishes began at around 6pm after protesters removed a burned-out truck on what is known as the Backwater Bridge, not far from the site where they have been camped out for weeks as they demonstrate against the pipeline.
The Morton County Sheriff's Department estimated that 400 protesters sought to cross the bridge on state Highway 1806.
A live stream early on Monday showed a continuing stand-off, with large lights illuminating smoke wafting across the scene.
The sheriff's department said in a statement at around 1am 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.
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At least one person was arrested.
Protesters said a gym in Cannon Ball was opened to aid demonstrators who were soaked on a night during which the temperature dipped into the low 20s, or were hit with tear gas.
Rema Loeb told the Associated Press that 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 sprayed with tear gas.
"It's been just horrible," said the 83-year-old, who travelled from Massachusetts about two weeks ago to join the protests.
The 1,200-mile (1,931km), 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 US dollars (£3.1 billion) pipeline has been the subject of months of protest 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 are also concerned that construction could threaten sacred sites.
Energy Transfer Partners has said no sites have been disturbed and that the pipeline will have safeguards against leaks, and is a safer method of transport for oil than rail or truck. The company has said the pipeline is largely complete except for the section under Lake Oahe.
The bridge lies near where protesters had set up camp on private property owned by the pipeline developer, Energy Transfer Partners, before they were forcibly removed by law enforcement officers on October 27. It is also about a mile from an uncompleted section under Lake Oahe, a Missouri River reservoir, where work has been on hold by order of federal agencies.
Tara Houska, an organiser with Honor The Earth, told the Bismarck Tribune that the Cannon Ball gym was opened to aid people who had been doused with water or tear gas.
The sheriff's department was not available for comment.
On Friday, Kelcy Warren, the chief executive of Dallas-based Energy Transfer Partners, said the company is unwilling to reroute the pipeline.
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