CANNON BALL, N.D. >> North Dakota officials are encouraging the hundreds of Dakota Access oil pipeline protesters to respect the mandate to leave the sprawling, months-old encampment that’s on federal land.

According to Standing Rock Sioux tribal leader Dave Archambault, the U.S. Army Corps of Engineers sent him a letter Friday that said all federal lands north of the Cannonball River will be closed to public access Dec. 5 for “safety concerns,” including the oncoming winter and the increasingly contentious clashes between protesters and police.

The largest encampment, Oceti Sakowin, is on Corps land in southern North Dakota, a place where several hundred people fighting against the four-state, $3.8 billion pipeline have created a self-sustaining community and put up semi-permanent structures in advance of the harsh winter.

The encampment is near the confluence of the Missouri and Cannonball rivers, and more than a mile from a Missouri River reservoir under which the pipeline will pass. That final large segment is yet to be completed, held up while the Corps consults with the tribe, who believe the project could harm the tribe’s drinking water and Native American cultural sites.

In early October, the Corps said it would not evict the encampment, which started as overflow from smaller private and permitted protest sites nearby and began growing in August, due to free speech reasons. Representatives from the Army Corps of Engineers didn’t immediately return multiple messages Friday or Saturday seeking comment and verification of the letter.

It isn’t clear whether the protesters will move voluntarily to another site or where that site might be; the Corps says in its letter that a free speech zone will be set up south of the Cannonball River. A news conference is planned later Saturday.

On Friday, Archambault, whose tribe offered protesters land on its reservation that’s south of the river earlier this fall, said “our resolve to protect our water is stronger than ever.”

President Barack Obama raised the possibility of rerouting the pipeline in that area earlier this month, something Kelcy Warren, CEO of Texas-based pipeline developer Energy Transfer Partners, told The Associated Press is not an option from the company’s standpoint. Obama said his administration is monitoring the “challenging situation” but would “let it play out for several more weeks.”

Some of the protests have resulted in violent confrontations and more than 500 people have been arrested since August. The Corps’ letter said that those who stay on the land after the deadline may be prosecuted, Archambault said.

But it’s the federal government’s job to peacefully close the camp because it allowed people to stay there in the first place, Gov. Jack Dalyrmple said in a statement Saturday.

“Our state and local law enforcement agencies continue to do all they can to keep private property and public infrastructure free from unpermitted protest activities, and its past time that the federal government provides the law enforcement resources … to enforce their own order to vacate,” the Republican said.

Republican U.S. Sen. John Hoeven and Democratic U.S. Sen. Heidi Heitkamp said the protesters need to move for public safety.

“The well-being and property of ranchers, farmers and everyone else living in the region should not be threatened by protesters who are willing to commit acts of violence,” Hoeven said in a statement Friday. He also called on the Obama administration to let work on the pipeline move forward, saying, “this difficult situation has gone on too long and we need to get it resolved.”

Heitkamp said the Corps’ order is “a needed step to support the safety of residents, workers, protesters and law enforcement.”