Protesters opposed to an oil pipeline planned to run beneath a lake near the Standing Rock Sioux reservation in North Dakota must vacate an area where thousands of demonstrators have been camped by Dec. 5 or face prosecution, according to U.S. authorities.

The U.S. Army Corps of Engineers, which manages the federal land where the main camp protesting the Dakota Access pipeline is located, said it would close public access to the area north of the Cannonball River, including to protesters, partly to protect the general public from violent confrontations between protesters and law enforcement that have occurred in the area.

Protest organizers said about 5,000 people are camped at the site. There are smaller camps on land not subject to the planned restrictions, including an area south of the Cannonball River where the Corps said it was establishing a free-speech zone.

On Saturday, North Dakota Governor Jack Dalrymple said he suppported the decision and the federal government must take the lead in any action to close encampments on Corps land.

As Dakota Pipeline Saga Drags On, Rancor Builds

Standing Rock Chairman Dave Archambault said he received notice on Friday about the decision in a letter from Colonel John Henderson, an Army Corps district commander.

“If they want public safety, the best thing the federal government could do is deny the easement” for the pipeline, Archambault told a news conference on Saturday. “We have an escalating situation where safety is a concern for everybody.”

Archambault said he saw the letter as notice the lands were no longer available for hunting, fishing and recreation, not to restrict First Amendment rights to free speech and the tribe was working on a location on reservation land should people choose to go there.

“I don’t think it will ever be an eviction where forces just come in and push people out,” Archambault said.

Demonstrators have protested for months against the $3.8 billion Dakota Access Pipeline, owned by Energy Transfer Partners LP , which is planned to carry Bakken shale oil from North Dakota to Illinois en route to U.S. Gulf Coast refineries.

Trump’s Stock in Dakota Access Pipeline Company Raises Concern

The 1,172-mile (1,885-km) project is mostly complete except for a segment planned to run under Lake Oahe less than half a mile north of Standing Rock.

The Obama administration in September postponed final approval of a permit required to allow tunneling beneath the lake, a move intended to give federal officials more time to consult with tribal leaders. But the delay also led to escalating tensions over the project.

Last weekend, police used water hoses in subfreezing weather in an attempt to disperse about 400 activists near the proposed tunnel excavation site.

Demonstrators plan a march at noon Sunday in Washington, from the Department of Justice to the Washington Monument.