NEAR THE STANDING ROCK SIOUX RESERVATION, N.D. (AP) — Opponents of a $3.8 billion Midwest oil pipeline are vowing to keep up the pressure after this week’s arrests at a construction zone in North Dakota.

The Standing Rock Sioux had quietly opposed the Dakota Access pipeline for months at a “spirit camp” near their reservation, which straddles the North Dakota-South Dakota border.

Protesters’ resistance heated up this week as at least 18 people were arrested, and a Hollywood actress joined their gathering.

Tribal members say the pipeline would disturb sacred sites and could affect drinking water on the reservation and for people downstream.

The four-state pipeline’s path in North Dakota would cross beneath the Little Missouri River once and the Missouri River twice. The company said the pipeline would include safeguards such as leak detection equipment.