Authorities are investigating an overnight fire that may have burned hundreds of acres near a Dakota Access pipeline protest camp in North Dakota.

Morton County sheriff's spokeswoman Donnell Preskey says the fire was reported early Sunday but quickly extinguished.

Preskey says the fire burned on private property, but declined to say where. She says a county commissioner estimated the fire burned about 400 acres.

Preskey says the cause is being investigated and no arrests have been made. She says two National Guard helicopters were sent to fight the blaze.

Protesters were evicted last week from property owned by the company building the pipeline. A company spokeswoman didn't immediately return a message seeking comment Sunday.

Demonstrators have been camping for months in an effort to block the nearly 1,200-mile, four-state pipeline.