BISMARCK, N.D. -- The acting secretary of the Army has ordered the Army Corps of Engineers to allow construction of the Dakota Access pipeline under a Missouri River reservoir, a North Dakota senator said, the latest twist in the months-long legal battle over the $3.8 billion project.

The Standing Rock Sioux, whose opposition to the project attracted the support of thousands of protesters from around the country to North Dakota, immediately vowed to return to court to stop it.

Sen. John Hoeven, a Republican, announced late Tuesday that Robert Speer directed the Army Corps of Engineers to "proceed" with an easement necessary to complete the pipeline. President Donald Trump signed an executive order signalling his support for the project a week ago.

On Wednesday, Army spokesman Maj. Gen. Malcolm Frost said the Army has begun its review.

"These initial steps do not mean the easement has been approved," Frost said.

Hoeven spokesman Don Canton said Speer's move means the easement "isn't quite issued yet, but they plan to approve it" within days.

The crossing under Lake Oahe, a wide section of the Missouri River in southern North Dakota, is the final big chunk of work on the pipeline designed to carry North Dakota oil through South Dakota and Iowa to a shipping point in Illinois.

The pipeline has been the target of months of protests led by the Standing Rock Sioux, whose reservation lies near the pipeline's route and who have argued that it's a threat to water and tribal artifacts.

The tribe has vowed to challenge any granting of the easement in court, and Chairman Dave Archambault renewed that vow Tuesday night.

"If it does become a done deal in the next few days, we'll take it to the judicial system," Archambault said. He added: "This is a good indicator of what this country is going to be up against in the next four years. So America has to brace itself."

The developer, Texas-based Energy Transfer Partners, said the pipeline would be safe.

An environmental assessment conducted last year determined the crossing would not have a significant impact on the environment. However, then-Assistant Army Secretary for Civil Works Jo-Ellen Darcy on Dec. 4 declined to issue an easement, saying a broader environmental study was warranted.

Energy Transfer Partners called Darcy's decision politically motivated and accused then-President Barack Obama's administration of delaying the matter until he left office. Two days before he left the White House, the Corps launched an environmental impact study of the crossing that could take up to two years to complete.

On Jan. 24, just four days after he took office, Trump signed an executive action telling the Corps to quickly reconsider the Dec. 4 decision.

The company appears poised to begin drilling under the lake immediately. Workers have already drilled entry and exit holes for the Oahe crossing, and the company has put oil in the pipeline leading up to the lake in anticipation of finishing the project, its executive vice-president Joey Mahmoud said in court documents filed earlier this month.

Hundreds and at times thousands of pipeline opponents calling themselves "water protectors" have camped on federal land near the crossing site since August, often clashing with police and prompting more than 625 arrests. The camp's population has thinned to fewer than 300 due to harsh winter weather and a plea by Archambault for the camp to disband before the spring flooding season.