The Standing Rock Sioux Tribe said on Tuesday it will take legal action against the U.S. Army's decision to grant the final easement that will allow the Dakota Access Pipeline to be completed.

In a statement, the tribe said the Army cannot circumvent a scheduled environmental impact study that was ordered in January. "The Army Corps lacks statutory authority to simply stop the EIS," they said.

The tribe has been in a months-long legal battle against Energy Transfer Partners, which is building the line.

The controversial project, which is meant to bring oil from North Dakota's Bakken shale region to Illinois, runs adjacent to the Standing Rock reservation in the southern part of North Dakota.