Thousands of people opposing the Dakota Access Pipeline have been camping for several months near the Cannonball River in North Dakota, but the Army Corps of Engineers says they will have to leave federal property north of the river by December 5.

North Dakota Gov. Jack Dalrymple says he supports that decision, but he says the federal government must take the lead in any action to close the land. Dalrymple cites public safety concerns and health risks due to camping in winter conditions.

The Corps says anyone on the property after December 5 will be trespassing and subject to prosecution.

Opponents of the four-state, $3.8 billion Dakota Access Pipeline, who call themselves "water protectors," say they do not intend to leave their encampment in the near future.

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Isaac Weston of the Ogala Sioux tribe from South Dakota says indigenous people are the wardens of the land, and the government cannot remove them. He says they have a right to be there, and they are protecting the land and water.

Others who spoke at a news conference Saturday in response to the Corps of Engineers' decision to close the land say they don't believe the Corps will force them off the land, but that the government's letter put the protesters on notice and limits the Corps' liability.

The letter was sent to the chairman of the Standing Rock Sioux tribe on Friday.

Dallas Goldtooth with the Indigenous Environmental Network says the government's apparent decision is "an atrocious example that colonization has not ended for us here as an indigenous people." He says he believes the decision to close the land will escalate tensions.

U.S. Sen. Heidi Heitkamp (D-North Dakota) says the Corps' decision is a needed step to keep residents, workers, protesters and authorities safe.

She issued a statement Saturday, saying that she has been pressing the White House to make a decision on an easement for the pipeline, and that the issue needs to be put to rest in the interest of public safety.

U.S. Sen. John Hoeven (R-North Dakota) is calling for the Obama Administration to approve the easement for the pipeline.