(CNN) Throngs of veterans from the group " Veterans Stand for Standing Rock " were arriving at the freezing Dakota Access Pipeline protest site on Sunday, one day before authorities are expected to remove protesters.

"We'll get settled on the fourth (December 4) and we'll begin actions with the morning ceremony on the fifth," one of the group leaders, Michael A. Wood Jr., said in a video posted on his verified Twitter account.

With a Monday showdown looming, the veterans will add a new dimension to the protest, which up to now has pitted members of the Sioux tribe and their supporters against local law enforcement officers.

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The US Army Corps of Engineers had warned that come Monday, activists who refuse to leave the campsite could be arrested, then backtracked, saying the agency has no plans to forcibly remove those who stay.

"By sending out a letter saying you have until December 5," Sioux Tribal Chairman Dave Archambault told CNN, " ... It just escalates and causes more concern for safety for everybody."

Photos: North Dakota pipeline protests Fireworks lit the sky at the Oceti Sakowin Camp near Cannon Ball, North Dakota on December 4, 2016, after the Army Corps of Engineers halted the Dakota Access Pipeline route. An executive order by President Donald Trump in January allowed work to resume. Hide Caption 1 of 32 Photos: North Dakota pipeline protests Activists embrace after the December halt of the Dakota Access Pipeline route. The $3.7 billion project that would cross four states and change the landscape of the US crude oil supply. The Standing Rock Sioux tribe says the pipeline would affect its drinking-water supply and destroy its sacred sites. Hide Caption 2 of 32 Photos: North Dakota pipeline protests An activist rides down from a ridge on the edge of the Standing Rock Sioux Reservation on December 4. Hide Caption 3 of 32 Photos: North Dakota pipeline protests Activists celebrate at Oceti Sakowin Camp on December 4. An executive order by President Donald Trump in January allows work to resume on the Dakota Access Pipeline, which the activists oppose. Hide Caption 4 of 32 Photos: North Dakota pipeline protests Dan Nanamkin of the Colville Nez Perce tribe drums a traditional song by the Cannonball River in Cannon Ball, North Dakota, on Thursday, December 1. Hide Caption 5 of 32 Photos: North Dakota pipeline protests A procession makes its way down to the Cannonball River to take part in a Native American water ceremony on December 1. Hide Caption 6 of 32 Photos: North Dakota pipeline protests Snow covers the camp on Wednesday, November 30. Hide Caption 7 of 32 Photos: North Dakota pipeline protests A person walks through snow and wind on Tuesday, November 29. Hide Caption 8 of 32 Photos: North Dakota pipeline protests People against the construction of the Dakota Access Pipeline speak at a news conference near Cannon Ball on Saturday, November 26. Hide Caption 9 of 32 Photos: North Dakota pipeline protests A sculpture stands at an encampment where protesters of the pipeline have been gathered for months. Hide Caption 10 of 32 Photos: North Dakota pipeline protests People protest along Highway 1806 as they walk past a sprawling encampment on Thursday, November 24. Hide Caption 11 of 32 Photos: North Dakota pipeline protests A man stands along Highway 1806 on November 24. Hide Caption 12 of 32 Photos: North Dakota pipeline protests In this image provided by the Morton County Sheriff's Department, law enforcement and protesters clash near the pipeline site on Sunday, November 20. Hide Caption 13 of 32 Photos: North Dakota pipeline protests Tonya Stands recovers after being pepper-sprayed by police on Wednesday, November 2. Stands was pepper-sprayed after swimming across a creek with other protesters hoping to build a new camp to block construction of the Dakota Access Pipeline. Hide Caption 14 of 32 Photos: North Dakota pipeline protests Dozens of protesters wade in cold creek waters as they confront local police on November 2. Hide Caption 15 of 32 Photos: North Dakota pipeline protests Tribe members make their way back to their camp on Saturday, October 29. Hide Caption 16 of 32 Photos: North Dakota pipeline protests Cousins Jessica and Michelle Decoteau take part in a protest outside the North Dakota state capitol in Bismarck on October 29. Hide Caption 17 of 32 Photos: North Dakota pipeline protests The burned hulks of heavy trucks sit on Highway 1806 on Friday, October 28, near a spot where Dakota Access Pipeline protesters were evicted a day earlier. Hide Caption 18 of 32 Photos: North Dakota pipeline protests Pipeline protesters sit in a prayer circle as a line of law enforcement officers make their way across the camp to relocate the protesters a few miles south on Thursday, October 27. Protesters had camped on private property. Hide Caption 19 of 32 Photos: North Dakota pipeline protests A protester is arrested as law enforcement surrounds the camp on October 27. Hide Caption 20 of 32 Photos: North Dakota pipeline protests Tires burn as armed soldiers and law enforcement officers stand in formation to force Dakota Access Pipeline protesters off the private land in Morton County. Hide Caption 21 of 32 Photos: North Dakota pipeline protests A protester shows where he was hit by a bean-bag round fired by officers trying to force protesters off the private land. Hide Caption 22 of 32 Photos: North Dakota pipeline protests JR American Horse leads a march to the pipeline site on Friday, September 9. Hide Caption 23 of 32 Photos: North Dakota pipeline protests Native Americans head to a rally at the state capitol in Denver on Thursday, September 8. They were showing their support for members of the Standing Rock Sioux tribe in North Dakota opposting the Dakota Access Pipeline. Hide Caption 24 of 32 Photos: North Dakota pipeline protests People hang a sign near what they say was sacred burial ground disturbed by bulldozers in Cannon Ball. Hide Caption 25 of 32 Photos: North Dakota pipeline protests Marlo Langdeau of the Lower Brule Sioux Tribe joins hundreds of Native Americans for a march near Cannon Ball on Sunday, September 4. Hide Caption 26 of 32 Photos: North Dakota pipeline protests Protesters march on September 4 in opposition to the Dakota Access Pipeline. Hide Caption 27 of 32 Photos: North Dakota pipeline protests Native Americans march to the site of a sacred burial ground on September 4. Hide Caption 28 of 32 Photos: North Dakota pipeline protests Native Americans ride with raised fists to the sacred burial ground on September 4 to protest the Dakota Access Pipeline. Hide Caption 29 of 32 Photos: North Dakota pipeline protests The Missouri River is seen beyond an encampment near Cannon Ball, where hundreds of people gathered to join the protest on September 4. Hide Caption 30 of 32 Photos: North Dakota pipeline protests Flags of Native American tribes from across the United States and Canada line the entrance to a protest encampment on Saturday, September 3. Hide Caption 31 of 32 Photos: North Dakota pipeline protests Phil Little Thunder Sr. attends an evening gathering at an encampment of Dakota Access Pipeline protesters on September 3. Hide Caption 32 of 32

Over the months of protest, the tribe has been joined by multiple groups and activists. They maintain that they have been peaceful, but at times violence has erupted.

The Morton County Sheriff's Office said protesters set fires while officers tried to disperse the crowds with tear gas, rubber bullets and water sprayed from hoses attached to fire engines. Just before Thanksgiving, on an evening when temperatures dropped below freezing, police sprayed protesters with a water cannon.

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Archambault said the accusations against the protesters are false, and that it's police who are being violent.

The protesters want to stop construction of the Dakota Access Pipeline, a $3.7 billion project that would transport 470,000 barrels of oil a day across four states.

It would pass through an oil-rich area in North Dakota where there's an estimated 7.4 billion barrels of undiscovered oil.

The Standing Rock Sioux tribe sued the US Army Corps of Engineers after the pipeline was granted final permits in July. The tribe says the project will not only threaten their environmental and economic well-being, but will also cut through land that is sacred.

Construction of it, they say, will "destroy our burial sites, prayer sites and culturally significant artifacts."