Thousands of gallons of oil spilled into creek near the Dakota Access protest camp

Clean-up is underway but could take until spring

The spill happened just 150 miles away from the protests (Getty Images)

Tens of thousands of gallons of crude oil have spilled into a creek around 150 miles from where protesters are camped in opposition to the Dakota Access pipeline.

A leak is thought to have contaminated the Ash Coulee Creek with an estimated 176,000 gallons of oil after part of the North Dakota hillside in which the pipeline was buried gave way.

The creek is roughly 150 miles from the epicenter of the Dakota Access pipeline protest camps, where members of the Standing Rock Sioux and other Native American tribes, as well as environmentalists from around the country, have demonstrated against the project on the grounds that it crosses beneath a lake that provides drinking water to native Americans.

They claim the route beneath Lake Oahe puts the water source in jeopardy and would destroy sacred land.

The proposed 1,172-mile-long Dakota Access pipeline would have sent roughly 470,000 barrels of crude oil under the Missouri River per day but the development was held up earlier this month when the US Army said it would not grant a key permit for construction.

Wendy Owen, a spokeswoman for True Companies which operates the Belle Fourchepipeline, said the leak was contained within hours of discovery on 5 December. Regarding the landslide, she told US broadcaster CNBC: "That is our number one theory, but nothing is definitive. We have several working theories and the investigation is ongoing."

It's not yet clear why electronic monitoring equipment didn't detect the leak, Ms Owen added.

Bill Suess, an environmental scientist from the North Dakota Health Department, said the spill migrated almost six miles from the original site along Ash Coulee Creek, and it fouled an unknown amount of private and US Forest Service land along the waterway.

The creek feeds into the Little Missouri River, but Mr Seuss said it appears no oil got that far and that no drinking water sources were threatened.

True Companies has a history of oil field-related spills in North Dakota and Montana, including a January 2015 pipeline break into the Yellowstone River.

The 32,000-gallon spill temporarily shut down water supplies in the downstream community of Glendive, Montana, after oil was detected in the city's water treatment system.

Federal pipeline safety regulators initiated 19 enforcement activities against the three True pipeline companies since 2004. Those resulted in $537,500 in proposed penalties, of which the company paid $397,200, according to Department of Transportation records.

About 60 workers are working on clean-up at Ash Coulee, which could last until spring, Ms Owen said.

US President-elect Donald Trump has given his backing to the Dakota Access pipeline, but the endorsement was nothing to do with his related investments, his campaign said.

He has a stake in the company building the pipeline, called Energy Transfer Partners, and he has a stake in Phillips 66, which holds a share of the project.

Kelcy Warren, CEO of Energy Transfer Partners, has given almost $169,000 to Mr Trump’s campaign and the Republican National Committee.

Additional reporting by Associated Press