A group of Democratic senators is asking President Obama to halt work on the controversial Dakota Access oil pipeline and conduct more thorough cultural and environmental reviews of the project before allowing it to go forward.

In a Thursday letter to Obama, the senators, led by former White House candidate Bernie Sanders Bernie SandersButtigieg stands in as Pence for Harris's debate practice Bernie Sanders warns of 'nightmare scenario' if Trump refuses election results Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE (I-Vt.), called the pipeline “a violation of tribal treaty rights,” and said federal regulators didn’t do a thorough enough assessment of its route before approving it.

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“We support the tribes along the pipeline route in their fight against the Dakota Access pipeline project,” the senators wrote.

They also tied the project to the Keystone XL pipeline, which last year saw Obama reject its permit request on environmental grounds. The senators said the oil that would travel through Dakota Access would contribute to climate change.

“It is imperative that the [Army] Corps’ permitting process be transparent and include public notice and participation, formal and meaningful tribal consultation, and adequate environmental review. Until that occurs, construction of this project must be halted.”

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The senators’ message comes days after a federal appeals court allowed construction of the 1,170-mile, $3.8 billion pipeline to move forward, ruling against the Standing Rock Sioux Tribe's request to temporarily block the project. The pipeline’s developers said they will undertake a “prompt resumption” of work.

The Army Corps and pipeline developers said they took pains to avoid tribal heritage sites in North Dakota. Another federal judge last month ruled against the tribe and gave his approval to the Army Corps’ permitting process.

Sanders was an early foe of the Dakota Access project, which has become a major issue for anti-fossil-fuel activists and environmentalists in Congress. A group of House Democrats has also asked Obama to stop construction on the project and do another assessment of it.

The White House has so far sidestepped questions on the matter, citing the legal challenges. The Obama administration is conducting an internal assessment of its permitting decisions for the project and is currently withholding an easement required for construction along a segment of the line to move forward.