CANNON BALL, N.D.  — Construction of the Dakota Access pipeline under a North Dakota reservoir has begun and the full pipeline should be operational within three months, the developer of the long-delayed project said Thursday, even as 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 Wednesday to lay pipe under Lake Oahe, clearing the way for completion of the 1,200-mile, $3.8 billion pipeline. ETP spokeswoman Vicki Granado confirmed early Thursday that construction resumed “immediately after receiving the easement.”

Workers had already drilled entry and exit holes for the crossing, and oil had been put in the pipeline leading up to the lake 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, as well as a prolonged court battle between the developer and the Army Corps of Engineers that oversees the federal land where the last segment of the pipeline is 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 Lake Oahe work while a lawsuit filed earlier by the two tribes against the pipeline proceeds. Attorney Nicole Ducheneaux said in court documents that the pipeline “will desecrate the waters upon which Cheyenne River Sioux tribal members rely.”