Protesters encamped for months on the North Dakota plains to block the completion of the Dakota Access Pipeline redoubled efforts Monday to prepare for a long, cold winter, even after the Army Corps of Engineers announced Sunday it would not approve an easement allowing the project's construction beneath a reservoir and across ancestral Sioux lands.

Hundreds of environmental activists, tribal members, celebrities and, as of last week, military veterans have camped at the site since the spring, facing off with police to block a pipeline that opponents allege will imperil cultural sites and the water supply for the Sioux tribe and others downstream.

The four-state, $3.8 billion pipeline would cross beneath a dammed section of the Missouri River, a reservoir called Lake Oahe. The project, stretching 1,172 miles from oil fields in western North Dakota to a terminal in Illinois, had obtained construction permits from federal and state officials and is about 90 percent complete.

This weekend, however, the Army Corps of Engineers, which is part of the Army, said it would not approve the easement needed to build the final major segment of the pipeline beneath the reservoir. Instead it is calling for an environmental study of alternative routes.

Activists embrace after learning Sunday that the Army Corps of Engineers would not grant an easement for the Dakota Access Pipeline. Helen H. Richardson/The Denver Post via Getty Images

"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," Assistant Secretary for Civil Works Jo-Ellen Darcy said in a statement. "The best way to complete that work responsibly and expeditiously is to explore alternate routes for the pipeline crossing."

The Army Corps of Engineers initially delayed granting an easement Nov. 14 pending further discussion with the Standing Rock Sioux Tribe, whose reservation is about a half-mile from the proposed pipeline crossing.

Environmental activists, tribal groups, left-leaning lawmakers and celebrities cheered the Army's decision, but warned that the fight is far from over. At the campsite, protesters continued building wood shelters and preparing for winter.

"The whole world is watching," Miles Allard, a member of the Standing Rock Sioux, told the Associated Press. "I'm telling all our people to stand up and not to leave until this is over."

He added: "We don't know what Trump is going to do."

President-elect Donald Trump has voiced support for the pipeline and other oil infrastructure projects, and he could take steps to greenlight the project's original route after taking office. He owns stock in the company that is building the pipeline, Energy Transfer Partner, as well as in a company that owns about a quarter of the pipeline, Phillips 66. His transition team has maintained that his support for the project is not a result of his investments.

An activist stands near a police barricade Sunday on the edge of the Standing Rock Sioux Reservation. Jim Watson/AFP/Getty Images

Trump's endorsement of Dakota Access "has nothing to do with his personal investments and everything to do with promoting policies that benefit all Americans," his transition team said in a memo Thursday that was obtained by the Associated Press and Reuters..

Energy Transfer Partners excoriated the Obama administration's decision, calling it "a purely political action."

"For more than three years now, Dakota Access Pipeline has done nothing but play by the rules," the company and Sunoco Logistics Partners said in a joint statement. "The White House's directive today to the Corps for further delay is just the latest in a series of overt and transparent political actions by an administration which has abandoned the rule of law in favor of currying favor with a narrow and extreme political constituency."

The months-long demonstration at the construction site has included an ongoing standoff with local law enforcement, which has led to some violent clashes between police and protesters. The Justice Department said it "will continue to monitor the situation in North Dakota."