Dakota Access oil pipeline activists have said they will not follow a federal-issued eviction notice to leave Standing Rock, despite threat of prosecution.
The US Army has ordered the camp established by protesters be closed to the public from December 5, citing federal regulations governing public lands.
Standing Rock Sioux tribal chairman Dave Archambault released the letter on Saturday (local time) from the district commander of the Corps of Engineers, John Henderson.
The letter says any "individuals found on Army land north of the river after December 5 would be considered trespassing and could be prosecuted" as a way to "protect the general public from the violent confrontations between protesters and law enforcement officials that have occurred in this area, and to prevent death, illness or serious injury" .
Those protesting the four-state, US$3.8 billion pipeline have created a self-sustaining community at the sprawling camp, building semi-permanent shelters and bringing in motorhomes.
On Sunday, local police deployed tear gas, "less-than-lethal" munitions and water cannons on hundreds of peaceful demonstrators amid sub-freezing temperatures.
Twenty-six people were hospitalised, and hundreds more were injured, The Guardian reported.