CANNON BALL, North Dakota — Protesters opposed to an oil pipeline project were sprayed with water from fire hoses in frigid temperatures after police say officers were pelted by flaming logs, rocks and other projectiles in a furious clash on a bridge.

The violent confrontation was the latest in a months-long protest over the Dakota Access Pipeline, which has pitted hundreds of Native American and environmental activists who say the $3.7 billion project threatens water resources against police guarding construction of the pipeline.

The standoff came to a head late Sunday, when police say hundreds of protesters forced their way onto the Backwater Bridge, which had been closed after numerous cars were burned on it in a protest in October that rendered it possibly unsafe.

As protesters moved to remove the burned out cars, police say numerous fires were set and debris hurled at cops manning a barricade on one edge of the bridge. Cops responded by firing tear gas and using hoses intended to put out the fires to repel protesters despite below-freezing temperatures..

“We are just not going to let people and protesters in large groups come in and threaten officers,” said Morton County Sheriff Kyle Kirchmeier. “In this circumstance it was the best option we had at that point.”

Protesters said police also fired rubber bullets at them. One activist was airlifted to Minneapolis after an explosion severely mangled her arm. Protesters said they believe the woman was wounded by a police concussion grenade, but cops denied being responsible and said it was possible the explosion was caused by an incendiary device intended to be thrown at police.

One protester was arrested while trying to push through the police barricade, Kirchmeier said, and 16 others were arrested the following morning. One officer was hospitalized after being struck in the head by a projectile, Kirchmeier said.

Protest leaders have said the closure of the bridge has been a point of frustration for weeks as many believed it is intended solely to keep demonstrators away from the pipeline construction zone.

“Folks have a right to be on a public road,” Dallas Goldtooth, of the Indigenous Environmental Network told the Bismarck Tribune. “It’s absurd that people who’ve been trying to take down the barricade now have their lives at risk.”

Kirchmeier said the bridge would not reopened until engineers had deemed it safe, which could not happen while the protests were ongoing nearby.

Completion of the pipeline, set to run approximately 1,200 miles from North Dakota to Illinois, was delayed in September so federal authorities could re-examine permits required by the Army Corps of Engineers.

Plans called for the pipeline to pass under Lake Oahe, a federally owned water source, and to skirt the Standing Rock Sioux Reservation by about half a mile. Most of the construction has otherwise been finished.

The Standing Rock tribe and environmental activists say the project would threaten water supplies and sacred Native American sites and ultimately contribute to climate change.