The acting secretary of the U.S. army has directed the Army Corps of Engineers to proceed with an easement necessary to complete the Dakota Access pipeline, North Dakota Senator John Hoeven said Tuesday.

Hoeven issued a statement late in the evening after he said acting army secretary Robert Speer informed him of the decision. Hoeven said he also spoke with U.S. Vice-President Mike Pence.

An army spokesman did not immediately respond to requests for comment Tuesday night. Hoeven's spokesman Don Canton says that 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 four-state, $3.8 billion US pipeline that will carry North Dakota oil through South Dakota and Iowa to Illinois.

U.S. President Donald Trump called on the Army Corps of Engineers last week to reconsider its December decision to withhold permission until more study is done on the crossing.

The Standing Rock Sioux has spent months protesting the project along with supporters from around the country. The tribe gets drinking water from the river and worries a pipeline leak would pollute the water. The developer, Texas-based Energy Transfer Partners, says the pipeline is safe.

On Tuesday evening, the Standing Rock tribe said the Army could not circumvent a scheduled environmental impact study that was ordered in January.

"The Army Corps lacks statutory authority to simply stop the EIS," they said in a statement.

Hundreds, and at times thousands, of pipeline opponents who have dubbed themselves "water protectors" have camped on federal land near the crossing site since last 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 Standing Rock Chairman Dave Archambault for the camp to disband before the spring flooding season.

"If it does become a done deal in the next few days, we'll take it to the judicial system," Archambault said Tuesday night.

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."

Company poised to drill immediately

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

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

Energy Transfer Partners 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.