NEW YORK/CANNON BALL, N.D. (Reuters) - Police fired tear gas and water at hundreds of protesters in the freezing North Dakota weather late Sunday and early Monday, in the latest violent clash over a pipeline project running through the state.

Dakota Access Pipeline protesters are seen at the Oceti Sakowin campground near the town of Cannon Ball, North Dakota, U.S. November 19, 2016 in an aerial photo provided by the Morton County Sheriff's Department. Morton County Sheriff's Department/Handout via REUTERS

An estimated 400 protesters mounted the Backwater Bridge just north of Cannon Ball, North Dakota, and attempted to force their way past police in what the Morton County Sheriff’s Department described as an “ongoing riot.”

The $3.7 billion Dakota Access project has drawn steady opposition from activists since the summer, led by the Standing Rock Sioux tribe, whose tribal lands are adjacent to the pipeline. Native American activists and environmentalists say the line threatens water resources and sacred tribal lands.

A joint statement from several activist groups said protesters Sunday were trying to remove burned vehicles blocking Backwater Bridge in order to restore access to the nearby Standing Rock Sioux encampments so emergency services and local traffic can move freely.

Police fired volleys of tear gas at the protesters to prevent them from crossing the bridge. Law enforcement also fired rubber bullets and sprayed protesters with water in temperatures that reached as low as 18 Fahrenheit (minus 8 Celsius) overnight.

“It is below freezing right now and the Morton County Sheriff’s Department is using a water cannon on our people - that is an excessive and potentially deadly use of force,” said Dallas Goldtooth, a spokesman for the Indigenous Environmental Network, one of the organizations involved in protests.

Completion of the pipeline, set to run 1,172 miles (1,885 km) from North Dakota to Illinois, was delayed in September so the U.S. Army Corps of Engineers could re-examine permits that would allow construction under the Missouri River, near to the tribe’s lands.

The main company behind the pipeline, Energy Transfer Partners LP ETP.N, is building the line to bring crude oil from North Dakota to Illinois en route to the Gulf Coast.

On an analyst call Monday to discuss ETP's merger with its sister company Sunoco Logistics Partners LP SXL.N, officials said they still expect final approval for the pipeline by the end of this year. Officials did not address the protests.

A statement from the sheriff’s’ department said one arrest had been made by 8:30 p.m. local time (0230 GMT Monday), about 2-1/2 hours after the incident began 45 miles (30 km) south of Bismarck, the North Dakota capital. About 100 to 200 protesters remained after midnight.

The Morton County Sheriff’s Department said officers on the scene of the latest confrontation were “describing protesters’ actions as very aggressive.”

Demonstrators tried to start about a dozen fires as they attempted to outflank and “attack” law enforcement barricades, the sheriff’s statement said. Police said protesters had hurled rocks and burning logs, striking one officer.

Dave Archambault, chairman of the Standing Rock Sioux tribe, said law enforcement is escalating the violence.

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“They say these are non-lethal weapons, but a water cannon in freezing weather is lethal. Using concussion grenades with tear gas can be lethal,” he said.

The latest confrontation began Sunday evening, after protesters attempted to remove a truck that had been on the bridge since Oct. 27, police said.

The North Dakota Department of Transportation closed the Backwater Bridge, which crosses Cantapeta Creek north of the Standing Rock Sioux tribe’s camp, after vehicles were burned on Oct. 27. The U.S. Army Corps of Engineers had also asked Morton County law enforcement to prevent protesters from trespassing on federal land.

Supporters of the pipeline said the project offers the most direct route for taking shale oil from North Dakota to Gulf Coast refineries and would be safer than road or rail transportation.

The Army Corps of Engineers last week said they will need more consultations with Native American tribes, even though the Corps said they had followed all legal requirements for permitting.

President-elect Donald Trump has not commented specifically on Dakota Access, but he has in the past been supportive of pipeline development. Should a decision be delayed to his term, the pipeline could be approved.