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James MacPherson, Associated Press
Native Americans protest the Dakota Access oil pipeline on Friday, Aug. 12, 2016 near the Standing Rock Sioux reservation in southern North Dakota. The Standing Rock Sioux Tribe went to court to try to block a $3.8 billion pipeline that's going in the ground fast to carry oil from North Dakota to Illinois. Now tribal members are trying to turn up the heat, with arrests in the pipeline construction zone near the 2.3-million acre reservation that straddles the North Dakota-South Dakota border.

NEAR THE STANDING ROCK SIOUX RESERVATION, N.D. — 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.