Washington, Nov 21 (IANS): Clashes between protesters and authorities over the Dakota Access Pipeline escalated after an estimated 400 people tried to breach a law enforcement barrier, US officials said.

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

However, the sheriff office's spokesman, Rob Keller, told NBC News that no water cannon were deployed.

"The water was being sprayed from a fire truck to control blazes set by activists," he said.

In a statement, the sheriff's office said tear gas and other "less than lethal means" were being used after protesters "engaged in organised 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 clash comes days after organisers held coordinated demonstrations across the country to protest a 1,170-mile oil pipeline that has generated the largest American-Indian protest movement in modern history.

Thousands of people have flocked to North Dakota, where the Standing Rock Sioux Tribe argues that the proposed pipeline could permanently contaminate its water source, the Missouri river.

Energy Transfer Partners, which is behind the pipeline, has said that it has taken measures to prevent such leaks and that the pipeline is far safer than transporting oil with trucks or trains, NBC News said.