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The company behind the controversial Dakota Access Pipeline said Wednesday that it has all the approvals it needs "to proceed expeditiously to complete construction" after the U.S. Army Corps of Engineers granted a crucial easement.

"We began drilling once we obtained the easement," a spokesperson for Energy Transfer Partners told NBC News Wednesday night.

The oil pipeline has been the scene of months of protests from those who fear the project could pollute the water supply for a Sioux reservation. Opponents also say the project could damage sites sacred to the Standing Rock Sioux.

The Army Corps said Wednesday that it granted the easement allowing the construction of a 30-inch pipeline on federally controlled lands at the Oahe Reservoir. The project is nearly finished, and the easement covers 1.25 miles of the pipeline that runs under the Missouri River.

"With this action, Dakota Access now has received all federal authorizations necessary to proceed expeditiously to complete construction of the pipeline," Energy Transfer Partners said in a statement Wednesday.

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The project was delayed during President Barack Obama's administration, and a review of environmental studies was launched after protests by the tribe and others, who call themselves "water protectors" instead of protesters.

Related: Army Corps of Engineers to Grant Final Permit for Pipeline

The chairman of the Standing Rock Sioux told Reuters that the tribe would continue to do whatever it can to oppose the project.

"We're running out of options, but that doesn't mean that it's over," Chairman David Archambault II told the news agency in a telephone interview. "We're still going to continue to look at all legal options available to us."

The Cheyenne River Sioux Tribe announced Wednesday that it would file a legal challenge to the granting of the easement, saying "it is illegal for the Trump Administration to sidestep the administrative process that was rightfully underway."

Crews remove waste from the opposition camp against the Dakota Access Pipeline near Cannon Ball, North Dakota, on Wednesday. Terray Sylvester / Reuters

Under Obama, the Army Corps and the Justice and Interior departments said in September that "this case has highlighted the need for a serious discussion on whether there should be nationwide reform with respect to considering tribes' views on these types of infrastructure projects."

Demonstrations against the project erupted in violent clashes at times, with equipment set on fire and allegations by protesters of excessive force used by authorities, including being doused with cold water in freezing temperatures. The Morton County Sheriff's Office said water wasn't used against protesters but was used to douse fires.

Related: U.S. Army Corps Gives Eviction Notice to Dakota Access Protest Camp

President Donald Trump said in a Jan. 24 memorandum that the pipeline served the national interest and ordered an approval review, which included the easement. The pipeline has been projected to transport about 470,000 barrels of oil a day, with a maximum capacity of 570,000 barrels a day.

The pipeline project spans 1,172 miles of new pipeline from North Dakota to Patoka, Illinois, running within a half-mile of the Standing Rock Sioux reservation. Another section of existing pipeline runs from Patoka to Nederland, Texas.

Energy Transfer Partners said in a statement Wednesday that it expects both sections to be in service by the second quarter of 2017.

Army Corps Col. John Henderson said in a statement Wednesday that "the safety of those located on Corps-managed land remains our top priority, in addition to preventing contaminants from entering the waterway."