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The Dakota Access pipeline developer said Monday that it has placed oil in the pipeline under a Missouri River reservoir in North Dakota and that it's preparing to put the pipeline into service.

Dallas-based Energy Transfer Partners made the announcement in a brief court filing with the U.S. District Court for the District of Columbia. The announcement marks a significant development in the long battle over the project that will move North Dakota oil 2,000 miles through South Dakota and Iowa to a shipping point in Illinois. The pipeline is three months behind schedule due to large protests and the objections of two American Indian tribes who say it threatens their water supply and cultural sites.

Related: Appeals Court Refuses to Stop Oil in Dakota Access Pipeline

ETP's filing did not say when the company expected the pipeline to be completely operating, and a spokeswoman did not immediately return an email seeking additional details.

"Oil has been placed in the Dakota Access Pipeline underneath Lake Oahe. Dakota Access is currently commissioning the full pipeline and is preparing to place the pipeline into service," the filing stated.

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Native American protesters and their supporters are confronted by security during a demonstration against construction of the Dakota Access Pipeline, near Cannon Ball, North Dakota, September 3, 2016. Robyn Beck / AFP/Getty Images

Despite the announcement, the battle isn't over. The Standing Rock and Cheyenne River Sioux tribes still have an unresolved lawsuit that seeks to stop the project. The Standing Rock chairman did not immediately return a call seeking comment on ETP's announcement.

The tribes argue that a rupture in the section that crosses under Lake Oahe would threaten their water supply and sacred sites and would prevent them from practicing their religion, which requires clean water.

The company disputes the tribes' claims and says the $3.8 billion pipeline is safe.

The tribes in December held up the project by successfully pushing the U.S. government for a full environmental study of the Lake Oahe crossing, which is in southern North Dakota. But the Army Corps of Engineers, which manages the Missouri River for the government, rescinded the study and gave the company permission to complete the pipeline at the urging of President Donald Trump shortly after he took office.

There were months of protests against the pipeline, mainly in North Dakota, where opponents set up a camp on Corps land between the Standing Rock Reservation and the pipeline route. At times it housed thousands of people, many of whom clashed with police, who made about 750 arrests between August and February. The on-the-ground protests waned after the Corps ordered the shutdown of the camp in February in advance of the spring flooding season.

While the protests have abated, opposition has not. The company on March 20 reported "recent coordinated physical attacks" on the pipeline without offering details. Authorities in South Dakota and Iowa confirmed that someone apparently used a torch to burn a hole through empty sections of the pipeline at aboveground shut-off valve sites.

North Dakota has become the second-biggest oil producer in the U.S. in the past decade, trailing only Texas. The state stands to gain more than $110 million annually in tax revenue with oil flowing through the pipe, according to an analysis by The Associated Press.

Associated Press writer Blake Nicholson in Bismarck, N.D., contributed to this report.

This story has been corrected to reflect that the filing was with the U.S. District Court for the District of Columbia, not the appeals court.