Protesters gather at an encampment on Saturday, a day after tribal leaders received a letter from the U.S. Army Corps of Engineers that told them the federal land would be closed to the public on Dec. 5, near Cannon Ball, N.D.

BISMARCK, N.D. — The U.S. 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 1,200-mile 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 $3.8 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.”

The agency previously had said it would not evict protesters due to free speech reasons.

The Corps’ letter came after residents in the area expressed feeling unsafe and frustrated with how the protest has swelled to scores of self-described “water protectors.” North Dakota officials contend the Corps should not allow people to be on the land without a permit.

More than 525 people from across the country have been arrested since August. In a recent clash between police and protesters, officers used tear gas, rubber bullets and large water hoses in freezing weather. Organizers said at least 17 protesters were taken to the hospital, some for hypothermia and one for a serious arm injury. One officer was injured.

North Dakota officials say the protest has cost the about $20 million in extra law enforcement costs.

The Oceti Sakowin camp is the overflow from smaller private and permitted protest sites nearby and began growing in August. It’s been called the largest gathering of Native American tribes in a century.

Standing Rock tribal members and other Native Americans believe the land with the encampment is rightly owned by the Sioux through a more than century-old treaty with the U.S. government.

Protest organizers said they don’t intend to leave or stop their acts of civil disobedience. Archambault and others don’t believe the Corps will forcibly evict people from the camp.