The four-state, $3.8 billion project is largely complete except for the now-blocked segment underneath Lake Oahe, a Missouri River reservoir. Assistant Secretary for Civil Works Jo-Ellen Darcy said in a news release that her decision was based on the need to "explore alternate routes" for the pipeline's crossing.
"Although we have had continuing discussion and exchanges of new information with the Standing Rock Sioux and Dakota Access, it's clear that there's more work to do," Darcy said. "The best way to complete that work responsibly and expeditiously is to explore alternate routes for the pipeline crossing."
The company constructing the pipeline, Dallas-based Energy Transfer Partners, had said it was unwilling to reroute the project. It and the Morton County Sheriff's Office, which has done much of the policing of the protests, didn't have immediate comment.
U.S. Secretary for the Interior Sally Jewell said in a statement that the Corps' "thoughtful approach ... ensures that there will be an in-depth evaluation of alternative routes for the pipeline and a closer look at potential impacts."
Standing Rock Sioux tribal chairman Dave Archambault didn't immediately respond to messages left seeking comment.
The federal government has ordered people to leave the Oceti Sakowin, or Seven Council Fires, encampment on Army Corps of Engineers' land by Monday. But demonstrators say they're prepared to stay, and authorities say they won't forcibly remove them.
Attorney General Loretta Lynch said the Department of Justice will "continue to monitor the situation" and stands "ready to provide resources to help all those who can play a constructive role in easing tensions."
"The safety of everyone in the area - law enforcement officers, residents and protesters alike - continues to be our foremost concern," she added.
Earlier Sunday, an organizer with Veterans Stand for Standing Rock said tribal elders had asked the military veterans not to have confrontations with law enforcement officials, adding the group is there to help out those who've dug in against the project.
About 250 veterans gathered about a mile from the main camp for a meeting with organizer Wes Clark Jr., the son of former Democratic presidential candidate Gen. Wesley Clark. The group had said about 2,000 veterans were coming, but it wasn't clear how many actually arrived.
"We have been asked by the elders not to do direct action," Wes Clark Jr. said. He then talked about North Dakota authorities' decision to move away from a key bridge north of the encampment by 4 p.m. Sunday if demonstrators agree to certain conditions, saying the National Guard and law enforcement have armored vehicles and are armed.
"If we come forward, they will attack us," Clark said. Instead, he told the veterans, "If you see someone who needs help, help them out."
Authorities moved a blockade from the north end of the Backwater Bridge with the conditions that protesters stay south of it and come there only if there is a prearranged meeting. 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.
"That heavy presence is gone now and I really hope in this de-escalation they'll see that, and in good faith.the leadership in those camps will start squashing the violent factions," Cass County Sheriff Paul Laney said in a statement. Protesters also are not supposed to walk, ride or fly drones north of the bridge, and Laney said that any violation will "will result in their arrest."
Veterans Stand for Standing Rock's GoFundMe.com page had raised more than $1 million of its $1.2 million goal by Sunday — money due to go toward food, transportation and supplies. Cars waiting to get into the camp Sunday afternoon were backed up for more than a half-mile.
"People are fighting for something, and I thought they could use my help," said Navy veteran and Harvard graduate student Art Grayson. The 29-year-old from Cambridge, Massachusetts, flew the first leg of the journey, then rode from Bismarck in the back of a pickup truck. He has finals this week, but told professors, "I'll see you when I get back."
Steven Perry, a 66-year-old Vietnam veteran who's a member of the Little Traverse Bay band of Odawa Indians in Michigan, spoke of one of the protesters' main concerns: that the pipeline could pollute drinking water. "This is not just a native issue," he said, "This is an issue for everyone."
Art Woodson and two other veterans drove 17 hours straight from Flint, Michigan, a city whose lead-tainted water crisis parallels with the tribe's fight over water, he said.
"We know in Flint that water is in dire need," the 49-year-old disabled Gulf War Army veteran said. "In North Dakota, they're trying to force pipes on people. We're trying to get pipes in Flint for safe water."
On Monday, some veterans will take part in a prayer ceremony in which they'll apologize for historical detrimental conduct by the military toward Native Americans and ask for forgiveness, Clark said. He also called the veterans' presence "about right and wrong and peace and love."