The US Army Corps of Engineers has set an early December deadline for American Indians and others to leave an encampment in North Dakota where they've been entrenched for months protesting the Dakota Access pipeline.
Tribes including the Standing Rock and Cheyenne River Sioux are fighting the Dakota Access project because they fear it will harm drinking water and cultural sites.
Texas-based pipeline developer Energy Transfer Partners says the 1900km pipeline through the Dakotas, Iowa and Illinois will be safe.
The Corps said last week in a letter to Standing Rock Sioux tribal leader Dave Archambault that all federal lands north of the Cannonball River will be closed to public access Dec. 5 for "safety concerns."
The order includes the sprawling encampment called Oceti Sakowin, or Seven Council Fires camp, that's a living protest against the four-state $5.4 billion pipeline.
The Corps cited North Dakota's oncoming winter and increasingly contentious clashes between protesters and police.
The agency says "it has no plans for forcible removal," but anyone on land north of the river after the deadline will be trespassing and may be prosecuted.
The Corps acknowledged that "more dangerous groups have joined this protest and are provoking conflict in spite of the public pleas from tribal leaders."
More than 525 people from across the country have been arrested since August.
In a recent clash between police and protesters near the path of the pipeline, officers used tear gas, rubber bullets and large water hoses in freezing weather.
The Standing Rock Sioux, whose reservation straddles the North Dakota-South Dakota border, are suing federal regulators for approving the pipeline.
They have challenged the Corps' decision to grant permits at more than 200 water crossings and argue that the pipeline would be placed less than a mile upstream of the reservation, potentially affecting drinking water for more than 8,000 tribal members and millions downstream.