Former Democratic presidential candidate Bernie Sanders and four other senators on Thursday called on President Barack Obama to order a comprehensive environmental review of a pipeline project that has stirred widespread opposition from Native Americans and environmental activists.

After a U.S. appeals court on Sunday night denied a request to halt construction of the Dakota Access Pipeline, the senators asked Obama to direct the Army Corps of Engineers to complete a full environmental impact statement for a contested part of the route that includes stronger tribal consultation.

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“The project’s current permits should be suspended and all construction stopped until a complete environmental and cultural review has been completed for the entire project,” said the letter by Sanders and Democratic Senators Dianne Feinstein, Ed Markey, Patrick Leahy and Benjamin Cardin.

In recent weeks, protests against the Dakota Access pipeline led by the Standing Rock Sioux tribe in North Dakota drew international attention, prompting the U.S. government to temporarily block its construction on federal land.

Tribal leaders say the pipeline will desecrate land and pollute water, especially around the planned crossing through Lake Oahe, a sacred site. Opposition to the pipeline has drawn support from 200 Native American tribes.

On Tuesday, anti-pipeline activists in four states closed pipeline valves to halt the flow of crude through arteries transporting 15 percent of U.S. oil consumption..

When fully connected, the 1,100-mile (1,770 km) pipeline would be the first to carry crude directly to the U.S. Gulf from the Bakken shale, a vast oil formation in North Dakota, Montana and parts of Canada.

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The $3.7 billion project is being built by the Dakota Access subsidiary of Dallas-based Energy Transfer Partners LP, which has vowed to complete construction.

“There must be a serious consideration of the full potential climate impacts of this pipeline prior to the Army Corps of Engineers approving any permits or easements for the Dakota Access pipeline,” the senators said.

Experts say that the full environmental review requested by the senators could take several months.

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The U.S. appeal court’s ruling was the second time the federal judiciary rejected the Standing Rock Sioux tribe’s request to halt construction of the pipeline. On Sept. 9, a U.S. judge rejected a similar request.

(Reporting By Valerie Volcovici)