By Stephanie Keith and Julia Harte

MANDAN, N.D./WASHINGTON (Reuters) - About 40 people involved in protests against the Dakota Access oil pipeline were arrested on Friday in rural North Dakota as the U.S. Army Corps of Engineers said it would soon clarify its plans for the controversial project near sacred tribal lands.

Army Corps of Engineers spokeswoman Amy Gaskill said it would issue a decision on its next steps within a few days, though it would perhaps not be a final decision. And it was unclear if the agency's next action would resolve the question of whether the line can be built as planned under Lake Oahe, where construction ceased in September.

In North Dakota, police confronted about 100 protesters barricading a highway adjacent to a construction easement for the pipeline, which has drawn steady opposition from Native American and environmental activists since the summer.

Within the pipeline easement itself, officers found construction equipment vandalized, with wires cut, machinery spray-painted and windows smashed, the Morton County Sheriff's Department reported.

A total of 37 arrests were made, most of them at a second illegal roadblock nearby, sheriff's spokesman Rob Keller said. Private security personnel reported activists hurled rocks at them, and one protester who attacked an officer was pepper-sprayed, according to police.

A spokesperson for Energy Transfer Partners, which owns the pipeline, confirmed two to three pieces of equipment were damaged, but the extent was not known.

Smoke was seen billowing from a large excavation machine near a site off Route 6 in rural North Dakota, and protesters had also climbed into other equipment, according to a Reuters witness. Two workers were seen leaving the scene.

Completion of the $3.7 billion Dakota Access Pipeline, set to run 1,172 miles (1,885 km) from North Dakota to Illinois, was delayed in September so federal authorities could re-examine permits required by the Army Corps of Engineers.

Plans called for the pipeline to pass under Lake Oahe, a federally owned water source, and to skirt the Standing Rock Sioux Reservation by about a half-mile (one km). Most of the construction has otherwise been completed.

The Standing Rock tribe and environmental activists said the project would threaten water supplies and sacred Native American sites and ultimately contribute to climate change. Protests have at times turned violent, as security dogs attacked activists during the summer and more recently as protesters set fire to vehicles and equipment.

“The tribe continues to ask water protectors to remain peaceful and prayerful," said Standing Rock Sioux spokeswoman Chelsea Hawkins on Friday.

'NOTHING ... TO STOP'

The Obama administration requested a voluntary halt to construction within 20 miles (32 km) of the lake on each side.

Energy Transfer Partners, which owns the line, continued to build to the edge of the federal land where the lake is located.

The company earlier this week said it was "mobilizing" drilling equipment to prepare to tunnel under the lake. That has angered protesters, who planned more protests in coming days.

An ETP spokeswoman said, "Construction is actually complete in North Dakota, except for the bore under the lake, so there is nothing for them to stop."

Pipeline supporters say the project offers the fastest and most direct route for bringing Bakken shale oil from North Dakota to U.S. Gulf Coast refineries and would be safer than transporting the oil by road or rail.

(Reporting by Stephanie Keith in Mandan, North Dakota; additional reporting by Liz Hampton in Houston and Steve Gorman in Los Angeles; Editing by Chizu Nomiyama and Cynthia Osterman)