The US Army has told Standing Rock campaigners that will close a large swathe of land they are camping on in protest against the building of the Dakota Access Pipeline. The Army Corps of Engineers said it will close the makeshift camp for "safety reasons" on 5 December.

The $3.7bn (£2.94bn) 1,200-mile pipeline is designed to transport crude oil from North Dakota into Illinois, and will also run through South Dakota and Iowa. Campaigners claim the project could harm drinking water and will encroach on sites sacred to Native Americans in the area.

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The US Army say that a "free speech zone" will be created south of the existing camp, on the Cannonball River. All access to the present camp on the north side of the river will be closed, it was announced on Friday, 25 November.

In past weeks, protesters have reported that they have been sprayed with water cannons during freezing weather, shot at with rubber bullets and tear gas, and they have been attacked with concussion grenades, injuring one protester's arm, according to reports.

Authorities claim demonstrators have blockaded roads and have attempted to set a vehicle on fire.

Tribal Chairman Dave Archambault II, the tribal Chairman of the Standing Rock Indian Reservation in North Dakota has urged defiance and called on outgoing US President Barack Obama to stop the construction.

"It is both unfortunate and ironic that this announcement comes the day after this country celebrates Thanksgiving – a historic exchange of goodwill between Native Americans and the first immigrants from Europe," Archambault said in a statement.

"Although the news is saddening, it is not at all surprising given the last 500 years of the treatment of our people. We have suffered much, but we still have hope that the president will act on his commitment to close the chapter of broken promises to our people and especially our children."

The US Army Corps of Engineers had pleaded directly to Archambault in a letter, with Colonel John Henderson, district commander of the Army Corps of Engineers saying: "I am asking you, as a tribal leader, to encourage members of your tribe, as well as any non-members who support you who are located in the encampments north of the Cannonball River on Corps' lands to immediately and peacefully move to the free speech zone south of the Cannonball River or to a more sustainable location for the winter."

Dakota Access pipeline
Police use tear gas on protesters occupying Turtle Island during a protest of the Dakota Access pipeline on the Standing Rock Indian Reservation near Cannon Ball, North Dakota Stephanie Keith/ Reuters

Henderson added that the move was to protect the public from violent confrontations between law enforcement and protesters, who label themselves as "water protectors".

Campaigners have been protesting at the Oceti Sakowin camp, named for the Seven Council Fires of the Sioux, since April attracting celebrity endorsements and demonstrators from around the US and the world. By some estimates, there are as many as 5,000 people living there.

Dakota Access Pipeline
Police use a water cannon to put out a fire started by protesters during a protest against plans to pass the Dakota Access pipeline near the Standing Rock Indian Reservation, near Cannon Ball, North Dakota Stephanie Keith/ Reuters