Many of those gathered at the encampment Monday vowed they would stay put until the four-state pipeline is scrapped. The group erected tents and teepees over the weekend, arguing that the land, which was recently purchased by the pipeline development company, rightfully belongs to Native Americans under a more than century-old treaty.
"We never ceded this land," Joye Braun, a protest organizer, said.
But the local sheriff's office called it trespassing.
A spokeswoman said the office wouldn't immediately remove the more than 100 people because it didn't have the manpower. Morton County Sheriff Kyle Kirchmeier said at a news conference Monday that authorities put out a call for help earlier this month and six states are sending officers. He would not say if the goal was to remove the protesters.
Safety remains the No. 1 priority, and authorities are attempting to negotiate with camp leaders, he said.
The development company, Texas-based Energy Transfer Partners, didn't return a request for comment Monday.
The $3.8 billion pipeline, most of which has been completed, crosses through North Dakota, South Dakota, Iowa and Illinois. Opponents worry about potential effects on drinking water on the Standing Rock Sioux Tribe's reservation and farther downstream on the Missouri River, as well as destruction of cultural artifacts, including burial sites.
On Monday, dozens of people were milling around the new protest site, some cooking over campfires.
Loren Bagola, who joined the protest from the Cheyenne River Reservation in South Dakota, said the demonstrators want to remain peaceful.
"We are here to pray for our ancestors that were desecrated and pray the pipeline people will find an alternative," he said. "We pray for their workers, too. We pray for police officers and their families. We all have one thing in common: We want clean drinking water."
But Vanessa Dundon, a Navajo from White Cone, Arizona, said the protesters were ready to use the hay bales and large logs stacked at the site to block the adjacent highway.
Protests have been going on in the area for months, including at a much larger encampment on U.S. Army Corps of Engineers' land about a half-mile away.
Demonstrators do not have a federal permit to be on the Corps' land, but the agency has said it wouldn't evict them due to free speech reasons. Authorities have criticized that decision, saying the site has been a launching point for protests at construction sites in the area.
In September, protesters and private security clashed after construction crews removed topsoil on the ranch. Authorities said four security guards and two guard dogs were injured; the tribe says protesters reported that six people had been bitten by security dogs, and at least 30 people were pepper-sprayed.
More than 260 people have been arrested since demonstrations began in August, nearly half of whom were arrested over the weekend during a large protest at a pipeline construction site.
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