Military vehicles are staged near the path of the Dakota Access pipeline Thursday, Feb. 9, 2017 near Cannon Ball, North Dakota. The developer says construction of the Dakota Access pipeline under a North Dakota reservoir has begun and that the full pipeline should be operational within three months. One of two tribes who say the pipeline threatens their water supply on Thursday filed a legal challenge asking a court to block construction while an earlier lawsuit against the pipeline proceeds. (AP Photo/James MacPherson)
Military vehicles are staged near the path of the Dakota Access pipeline Thursday, Feb. 9, 2017 near Cannon Ball, North Dakota. The developer says construction of the Dakota Access pipeline under a North Dakota reservoir has begun and that the full pipeline should be operational within three months. One of two tribes who say the pipeline threatens their water supply on Thursday filed a legal challenge asking a court to block construction while an earlier lawsuit against the pipeline proceeds. (AP Photo/James MacPherson) Credit: ap photograph

Cannon Ball, n.d. — Construction crews have resumed work on the final segment of the Dakota Access pipeline, and the developer of the long-delayed project said on Thursday that the full system could be operational within three months.

Meanwhile, an American Indian tribe filed a legal challenge to block the work and protect its water supply.

The Army granted Energy Transfer Partners formal permission on Wednesday to lay pipe under a North Dakota reservoir, clearing the way for completion of the 1,200-mile pipeline. Company spokeswoman Vicki Granado confirmed early Thursday that construction began “immediately after receiving the easement.”

Workers had already drilled entry and exit holes for the segment, and oil had been put in the pipeline leading up to Lake Oahe in anticipation of finishing the project.

“The estimate is 60 days to complete the drill and another 23 days to fill the line to Patoka,” Granado said, referring to the shipping point in Illinois that is the pipeline’s destination.

Work was stalled for months due to opposition by the Standing Rock Sioux and Cheyenne River Sioux tribes. Both tribes argue that the pipeline threatens their water supply and cultural sites.

The Cheyenne River reservation in South Dakota borders the Standing Rock reservation, which straddles the North Dakota-South Dakota border. The last piece of the pipeline is to pass under the lake on the Missouri River, which marks the eastern border of both reservations.

A separate court battle unfolded between the developer and the Army Corps of Engineers, which oversees the federal land where the last segmentis now being laid. President Donald Trump last month instructed the Corps to advance pipeline construction.

The Cheyenne River Sioux on Thursday asked a federal judge to stop the work while a lawsuit filed earlier by the tribes proceeds.