Protesters have been gathered for months, despite freezing temperatures.
Amnesty International has urged Barack Obama to halt the construction of a controversial oil pipeline in North Dakota and condemned possible instances of “excessive use of force” against largely peaceful protesters.
Days after authorities in the state were criticised for using water cannons against protesters in sub-zero temperatures, the human rights group wrote to the president to allege an “over-militarized” response by police at Standing Rock.
“Amnesty International urges you to halt construction of the Dakota Access Pipeline and ensure that the human rights of indigenous people and others opposed to the pipeline are respected, protected and fulfilled – including their right to peaceful protest and assembly,” the group said.
“We have sent four human rights observer teams to Standing Rock and have documented an over-militarisation of law enforcement in response to largely peaceful protests and possible instances of excessive use of force.”
Protesters have for months been demonstrating against the construction of the North Dakota Access Pipeline, a 1,200-mile, four-state project designed to carry oil from western North Dakota to a shipping point in Illinois.
Members of the Standing Rock Sioux, whose reservation lies near the pipeline route, have opposed a proposal for the $3.8bn project to pass under the Missouri River at a point close to their homes. They argue that with 470,000 barrels of crude oil passing beneath the river every day, a leak would contaminate their drinking water supply.
Amnesty’s letter to Mr Obama followed a previous request to the US Justice Department that it review the way local police are treating protesters, whose demonstrations have been largely peaceful.
Writing on Medium, Zeke Johnson of Amnesty’s Individuals at Risk Programme, said he had visited the protest site four times.
“What I’ve seen there and on video has deeply concerned me. Non-violent Indigenous People opposed to the Dakota Access pipeline have been met with over-militarised policing and excessive, disproportionate and unnecessary military force,” he said.
“People exercising their human rights to assemble, pray and speak out have been brutally arrested, shot with rubber bullets, drenched in tear gas, and sprayed with water in freezing temperatures.”
On Sunday, in a sign of possible compromise between protesters and North Dakota officials, state authorities said they would move from a key bridge near the main protest camp, if demonstrators agreed to certain conditions.
The Associated Press said the Morton County Sheriff’s Office said it had made the offer after discussions with military veterans who have travelled to the site to build shelters for protesters.
“The question was asked if we would consider pulling back from the Backwater Bridge,” said Sheriff Paul Laney. “And the answer is yes. We want this to de-escalate.”
Authorities will move from the north end of the Backwater Bridge by 4pm on Sunday, they said, if protesters stayed south of the bridge in the Oceti Sakowin camp, where thousands are camped out.
Authorities also asked protesters not to remove barriers on the bridge, which they have said was damaged in the late October conflict that led to several people being hurt, including a serious arm injury.
Hundreds of veterans are due to gather on Sunday at the reservation, then go to the main camp.
Veterans Stand for Standing Rock spokeswoman Ashleigh Jennifer Parker told AP the group’s mission was “to go and ask and offer if we can help and support the tribes that are already there”.