The activists, who are demonstrating in sub-zero temperatures, have been ordered to leave the area by Monday.
It is unclear if they will obey.
The pipeline, which runs close to the Standing Rock Sioux Native American Indian reservation, is nearly complete except for a section running underneath a nearby river.
President-elect Donald Trump has said he supports the completion of the pipeline - he has stocks in Texas-based Energy Transfer Partners, the project's builder, and Phillips 66, which owns one-quarter of the pipeline.
