U.S. president-elect Donald Trump isn't saying what he'll do about the $3.8 billion US, four-state Dakota Access oil pipeline once he takes office in January.

Trump spokesman Jason Miller said Monday that the incoming president supports construction of the pipeline. But he wouldn't say whether Trump would reverse an Army Corps of Engineers decision to deny a permit for the pipeline to cross under a Missouri River reservoir in southern North Dakota.

The segment under Lake Oahe is the only remaining big chunk of construction on the 1,900-kilometre pipeline to carry North Dakota oil through the Dakotas and Iowa to a shipping point in Illinois. The Standing Rock Sioux says the project threatens cultural sites and drinking water on its nearby reservation.

Texas-based pipeline developer Energy Transfer Partners says the Corps' decision was politically motivated. The company continues to seek permission for the crossing from a federal judge.

Miller says the Trump administration will review the situation "and make the appropriate determination."

'Top priority'

A trade association representing the U.S. oil and natural gas industry on Monday urged Trump to make approval of the Dakota Access oil pipeline a "top priority" when he takes office next month.

American Petroleum Institute President and CEO Jack Gerard said in a statement that the Obama administration "is putting politics over sound public policy." He said Trump should "stand up for American consumers and American workers."

Meanwhile, the leader of the Standing Rock Sioux said the tribe "will be forever grateful to the Obama administration" for the Army Corps of Engineers' decision to refuse to allow the construction of a pipeline

Chairman Dave Archambault says he hopes Energy Transfer Partners and the incoming Trump administration will respect that decision.

Archambault said the Corps' decision "took tremendous courage."