Construction of the Dakota Access oil pipeline will proceed, though the immediate timeline remains unclear, a state senator said.

U.S. Sen. Heidi Heitkamp, D-N.D., said she was informed that the acting secretary of the Army has directed the Army Corps of Engineers to sign off on the final easements to complete the construction of the oil pipeline.

"We know construction will move forward -- though we are waiting on more information in regards to a timeline for when construction can begin," she said in a statement.

Most of the pipeline is completed, save for a section that would run under the state's Lake Oahe. That section has been the source of long-standing protests from the environmental community and aboriginal groups concerned about water quality and the sanctity of cultural sites.

The Standing Sioux Rock tribe, the group at the center of the standoff over the pipeline, said it was disappointed, but not surprised, by the decision. From its perspective, the order from the Army is not a formal issuance of an easement, but a notice that one is imminent.

U.S. President Donald Trump last week issued an executive memoranda calling for the expedited clearance of hurdles for North American energy infrastructure like the Dakota Access pipeline. By tribal readings, that order means Congress still needs formal notification before construction can actually proceed.

"If and when the easement is granted, the Standing Rock Sioux Tribe will vigorously pursue legal action," the tribe said in a statement.

The Army Corps of Engineers in the final weeks of the previous administration said it would not approve an easement for further construction because of water-quality concerns. Sen. John Hoeven, R-N.D., offered assurances in a statement saying the project would move forward in an environmentally sound fashion.

Dakota Access would increase pipeline transit capacity for a region that depends in part on rail to take out of the area. Rail is more expensive than pipeline transit and carries unique safety risks of its own.