Governor cites harsh winter conditions in the area in ordering a mandatory evacuation.
North Dakota's governor has ordered an "emergency evacuation" of thousands of activists camped on federal property near a pipeline project they are trying to halt in the oil-rich US state.
Governor Jack Dalrymple cited hazards posed by harsh winter weather in the area in ordering a mandatory evacuation, though he did not specify how he intended to enforce the measure.
Dalrymple's order comes days after the US Army Corps of Engineers, which manages the site, said it would give the demonstrators until December 5 to vacate their encampment, about 45 miles (72 km) south of Bismarck, the state capital.
Last week, local law enforcement sought to disperse the Native American and environmental activists by spraying them with water in sub-freezing temperatures.
A spokeswoman for the Standing Rock Sioux Tribe, whose reservation lies about a half mile (0.8 km) from the proposed route of the Dakota Access Pipeline, did not have an immediate comment.
But activists who have spent months protesting plans to route the oil pipeline beneath a lake near the Standing Rock reservation have vowed to continue their resistance to a project they say poses a threat to water resources and sacred Native American sites.
North Dakota governor's office said the order was effective immediately and would remain in force "until rescinded."
"Winter conditions have the potential to endanger human life, especially when they are exposed to these conditions without proper shelter, dwellings or sanitation for prolonged periods of time," the order stated.
Some protest leaders scoffed at the order, suggesting a forced evacuation of the camp could prove even more dangerous to the activists.
"We're in the heart of winter now. To even think of a forced removal is terrifying," said Dallas Goldtooth, an organizer with Indigenous Environmental Network, who estimated there were around 5,000 people in the camp.
"They don't need to be worried about us in the winter," said Kandi Mossett, another network organizer. "We're perfectly capable of being self-sufficient. So using this as an excuse is insulting."
Morton County Sheriff Kyle Kirchmeier added to the pressure on Monday by issuing a video statement urging protesters to avoid subjecting themselves to "life-threatening conditions" by remaining exposed to the elements with little shelter.
The National Weather Service has posted a winter storm warning for most of western and central North Dakota, forecasting the possibility of heavy snow through Wednesday.
The $3.8 billion, 1,172-mile (1,885-km) pipeline project is mostly complete except for a segment that is supposed to run under Lake Oahe, a reservoir formed by a dam on the Missouri River.
The Obama administration in September postponed final approval of an Army Corps permit required to allow tunneling beneath the lake, a move intended to give federal officials more time to consult with tribal leaders. The delay also led to escalating tensions over the project.
The companies say the pipeline would carry Bakken shale oil more cheaply and safely from North Dakota to Illinois en route to U.S. Gulf Coast refineries than it could be shipped by railroad or tanker trucks.