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.

Sioux from Standing Rock claim victory over Dakota Pipeline Show all 21 1 /21 Sioux from Standing Rock claim victory over Dakota Pipeline Sioux from Standing Rock claim victory over Dakota Pipeline CANNON BALL, ND - DECEMBER 05: Despite blizzard conditions, military veterans march in support of the "water protectors" at Oceti Sakowin Camp on the edge of the Standing Rock Sioux Reservation on December 5, 2016 outside Cannon Ball, North Dakota. Over the weekend a large group of military veterans joined native Americans and activists from around the country who have been at the camp for several months trying to halt the construction of the Dakota Access Pipeline. Yesterday the US Army Corps of Engineers announced that it will not grant an easement for the pipeline to cross under a lake on the Sioux Tribes Standing Rock reservation. The proposed 1,172-mile-long pipeline would transport oil from the North Dakota Bakken region through South Dakota, Iowa and into Illinois. (Photo by Scott Olson/Getty Images) Getty Images Sioux from Standing Rock claim victory over Dakota Pipeline CANNON BALL, ND - DECEMBER 04: Fireworks fill the night sky above Oceti Sakowin Camp as activists celebrate after learning an easement had been denied for the Dakota Access Pipeline near the edge of the Standing Rock Sioux Reservation on December 4, 2016 outside Cannon Ball, North Dakota. The US Army Corps of Engineers announced today that it will not grant an easement to the Dakota Access Pipeline to cross under a lake on the Sioux Tribes Standing Rock reservation, ending a months-long standoff. (Photo by Scott Olson/Getty Images) Getty Images Sioux from Standing Rock claim victory over Dakota Pipeline CANNON BALL, ND - DECEMBER 05: Despite blizzard conditions, military veterans march in support of the "water protectors" at Oceti Sakowin Camp on the edge of the Standing Rock Sioux Reservation on December 5, 2016 outside Cannon Ball, North Dakota. Over the weekend a large group of military veterans joined native Americans and activists from around the country who have been at the camp for several months trying to halt the construction of the Dakota Access Pipeline. Yesterday the US Army Corps of Engineers announced that it will not grant an easement for the pipeline to cross under a lake on the Sioux Tribes Standing Rock reservation. The proposed 1,172-mile-long pipeline would transport oil from the North Dakota Bakken region through South Dakota, Iowa and into Illinois. (Photo by Scott Olson/Getty Images) Getty Images Sioux from Standing Rock claim victory over Dakota Pipeline CANNON BALL, ND - DECEMBER 04: Fireworks fill the night sky above Oceti Sakowin Camp as activists celebrate after learning an easement had been denied for the Dakota Access Pipeline near the edge of the Standing Rock Sioux Reservation on December 4, 2016 outside Cannon Ball, North Dakota. The US Army Corps of Engineers announced today that it will not grant an easement to the Dakota Access Pipeline to cross under a lake on the Sioux Tribes Standing Rock reservation, ending a months-long standoff. (Photo by Scott Olson/Getty Images) Getty Images Sioux from Standing Rock claim victory over Dakota Pipeline CANNON BALL, ND - DECEMBER 05: Despite blizzard conditions, military veterans march in support of the "water protectors" at Oceti Sakowin Camp on the edge of the Standing Rock Sioux Reservation on December 5, 2016 outside Cannon Ball, North Dakota. Over the weekend a large group of military veterans joined native Americans and activists from around the country who have been at the camp for several months trying to halt the construction of the Dakota Access Pipeline. Yesterday the US Army Corps of Engineers announced that it will not grant an easement for the pipeline to cross under a lake on the Sioux Tribes Standing Rock reservation. The proposed 1,172-mile-long pipeline would transport oil from the North Dakota Bakken region through South Dakota, Iowa and into Illinois. (Photo by Scott Olson/Getty Images) Getty Images Sioux from Standing Rock claim victory over Dakota Pipeline CANNON BALL, ND - DECEMBER 04: Native American and other activists celebrate after learning an easement had been denied for the Dakota Access Pipeline at Oceti Sakowin Camp on the edge of the Standing Rock Sioux Reservation on December 4, 2016 outside Cannon Ball, North Dakota. The US Army Corps of Engineers announced today that it will not grant an easement to the Dakota Access Pipeline to cross under a lake on the Sioux Tribes Standing Rock reservation, ending a months-long standoff. (Photo by Scott Olson/Getty Images) Getty Sioux from Standing Rock claim victory over Dakota Pipeline CANNON BALL, ND - DECEMBER 04: Chief Arvol Looking Horse of the Lakota/Dakota/Nakota Nation listens to speakers during an interfaith ceremony at Oceti Sakowin Camp on the edge of the Standing Rock Sioux Reservation on December 4, 2016 outside Cannon Ball, North Dakota. Native Americans and activists from around the country have been gathering at the camp for several months trying to halt the construction of the Dakota Access Pipeline. Today the US Army Corps of Engineers announced that it will not grant an easement for the pipeline to cross under a lake on the Sioux Tribes Standing Rock reservation, ending the months-long standoff. The proposed 1,172-mile-long pipeline would transport oil from the North Dakota Bakken region through South Dakota, Iowa and into Illinois. (Photo by Scott Olson/Getty Images) Getty Images Sioux from Standing Rock claim victory over Dakota Pipeline CANNON BALL, ND - DECEMBER 04: An Native American activist rides down fom a ridge which overlooks Oceti Sakowin Camp on the edge of the Standing Rock Sioux Reservation on December 4, 2016 outside Cannon Ball, North Dakota. Native Americans and activists from around the country have been gathering at the camp for several months trying to halt the construction of the Dakota Access Pipeline. The proposed 1,172-mile-long pipeline would transport oil from the North Dakota Bakken region through South Dakota, Iowa and into Illinois. (Photo by Scott Olson/Getty Images) Getty Images Sioux from Standing Rock claim victory over Dakota Pipeline CANNON BALL, ND - DECEMBER 05: Despite blizzard conditions, military veterans march in support of the "water protectors" at Oceti Sakowin Camp on the edge of the Standing Rock Sioux Reservation on December 5, 2016 outside Cannon Ball, North Dakota. Over the weekend a large group of military veterans joined native Americans and activists from around the country who have been at the camp for several months trying to halt the construction of the Dakota Access Pipeline. Yesterday the US Army Corps of Engineers announced that it will not grant an easement for the pipeline to cross under a lake on the Sioux Tribes Standing Rock reservation. The proposed 1,172-mile-long pipeline would transport oil from the North Dakota Bakken region through South Dakota, Iowa and into Illinois. (Photo by Scott Olson/Getty Images) Getty Images Sioux from Standing Rock claim victory over Dakota Pipeline CANNON BALL, ND - DECEMBER 04: Political activist Cornel West listen to speakers during an interfaith ceremony at Oceti Sakowin Camp on the edge of the Standing Rock Sioux Reservation on December 4, 2016 outside Cannon Ball, North Dakota. Native Americans and activists from around the country have been gathering at the camp for several months trying to halt the construction of the Dakota Access Pipeline. Today the US Army Corps of Engineers announced that it will not grant an easement for the pipeline to cross under a lake on the Sioux Tribes Standing Rock reservation, ending the months-long standoff. The proposed 1,172-mile-long pipeline would transport oil from the North Dakota Bakken region through South Dakota, Iowa and into Illinois. (Photo by Scott Olson/Getty Images) Getty Images Sioux from Standing Rock claim victory over Dakota Pipeline Activists hold hands during a prayer circle as they try to surround the entire camp at Oceti Sakowin Camp on the edge of the Standing Rock Sioux Reservation on December 4, 2016 outside Cannon Ball, North Dakota. Native Americans and activists from around the country gather at the camp trying to halt the construction of the Dakota Access Pipeline. / AFP / JIM WATSON (Photo credit should read JIM WATSON/AFP/Getty Images) AFP/Getty Images Sioux from Standing Rock claim victory over Dakota Pipeline Activists celebrate at Oceti Sakowin Camp on the edge of the Standing Rock Sioux Reservation on December 4, 2016 outside Cannon Ball, North Dakota, after hearing that the Army Corps of Engineers has denied the current route for the Dakota Access pipeline. / AFP / JIM WATSON (Photo credit should read JIM WATSON/AFP/Getty Images) AFP/Getty Images Sioux from Standing Rock claim victory over Dakota Pipeline CANNON BALL, ND - DECEMBER 04: Chief Arvol Looking Horse (L) of the Lakota/Dakota/Nakota Nation listens as political activist Cornel West speaks during an interfaith ceremony at Oceti Sakowin Camp on the edge of the Standing Rock Sioux Reservation on December 4, 2016 outside Cannon Ball, North Dakota. Native Americans and activists from around the country have been gathering at the camp for several months trying to halt the construction of the Dakota Access Pipeline. Today the US Army Corps of Engineers announced that it will not grant an easement for the pipeline to cross under a lake on the Sioux Tribes Standing Rock reservation, ending the months-long standoff. The proposed 1,172-mile-long pipeline would transport oil from the North Dakota Bakken region through South Dakota, Iowa and into Illinois. (Photo by Scott Olson/Getty Images) Getty Images Sioux from Standing Rock claim victory over Dakota Pipeline CANNON BALL, ND - DECEMBER 05: Military veterans from Southern California collect firewood for their campsite at Oceti Sakowin Camp on the edge of the Standing Rock Sioux Reservation on December 5, 2016 outside Cannon Ball, North Dakota. Over the weekend a large group of military veterans joined native Americans and activists from around the country who have been at the camp for several months trying to halt the construction of the Dakota Access Pipeline. Yesterday the US Army Corps of Engineers announced that it will not grant an easement for the pipeline to cross under a lake on the Sioux Tribes Standing Rock reservation. The proposed 1,172-mile-long pipeline would transport oil from the North Dakota Bakken region through South Dakota, Iowa and into Illinois. (Photo by Scott Olson/Getty Images) Getty Images Sioux from Standing Rock claim victory over Dakota Pipeline CANNON BALL, ND - DECEMBER 05: Despite blizzard conditions, military veterans march in support of the "water protectors" at Oceti Sakowin Camp on the edge of the Standing Rock Sioux Reservation on December 5, 2016 outside Cannon Ball, North Dakota. Over the weekend a large group of military veterans joined native Americans and activists from around the country who have been at the camp for several months trying to halt the construction of the Dakota Access Pipeline. Yesterday the US Army Corps of Engineers announced that it will not grant an easement for the pipeline to cross under a lake on the Sioux Tribes Standing Rock reservation. The proposed 1,172-mile-long pipeline would transport oil from the North Dakota Bakken region through South Dakota, Iowa and into Illinois. (Photo by Scott Olson/Getty Images) Getty Images Sioux from Standing Rock claim victory over Dakota Pipeline CANNON BALL, ND - DECEMBER 03: Activists participate in an art project conceived by Cannupa Hunska Luger, from the Standing Rock Sioux Tribe, at Oceti Sakowin Camp on the edge of the Standing Rock Sioux Reservation on December 3, 2016 outside Cannon Ball, North Dakota. Native Americans and activists from around the country have been gathering at the camp for several months trying to halt the construction of the Dakota Access Pipeline. The proposed 1,172-mile-long pipeline would transport oil from the North Dakota Bakken region through South Dakota, Iowa and into Illinois. (Photo by Scott Olson/Getty Images) Getty Images Sioux from Standing Rock claim victory over Dakota Pipeline CANNON BALL, ND - DECEMBER 05: Military veterans are briefed on cold-weather safety issues and their overall role at Oceti Sakowin Camp on the edge of the Standing Rock Sioux Reservation on December 5, 2016 outside Cannon Ball, North Dakota. Over the weekend a large group of military veterans joined native Americans and activists from around the country who have been at the camp for several months trying to halt the construction of the Dakota Access Pipeline. Yesterday the US Army Corps of Engineers announced that it will not grant an easement for the pipeline to cross under a lake on the Sioux Tribes Standing Rock reservation. The proposed 1,172-mile-long pipeline would transport oil from the North Dakota Bakken region through South Dakota, Iowa and into Illinois. (Photo by Scott Olson/Getty Images) Getty Images Sioux from Standing Rock claim victory over Dakota Pipeline CANNON BALL, ND - DECEMBER 04: Native American and other activists celebrate after learning an easement had been denied for the Dakota Access Pipeline at Oceti Sakowin Camp on the edge of the Standing Rock Sioux Reservation on December 4, 2016 outside Cannon Ball, North Dakota. The US Army Corps of Engineers announced today that it will not grant an easement to the Dakota Access Pipeline to cross under a lake on the Sioux Tribes Standing Rock reservation, ending a months-long standoff. (Photo by Scott Olson/Getty Images) Getty Images Sioux from Standing Rock claim victory over Dakota Pipeline CANNON BALL, ND - DECEMBER 04: Native American activists celebrate after learning an easement had been denied for the Dakota Access Pipeline at Oceti Sakowin Camp on the edge of the Standing Rock Sioux Reservation on December 4, 2016 outside Cannon Ball, North Dakota. The US Army Corps of Engineers announced today that it will not grant an easement to the Dakota Access Pipeline to cross under a lake on the Sioux Tribes Standing Rock reservation, ending a months-long standoff. (Photo by Scott Olson/Getty Images) Getty Images Sioux from Standing Rock claim victory over Dakota Pipeline CANNON BALL, ND - DECEMBER 04: Native American and other activists celebrate after learning an easement had been denied for the Dakota Access Pipeline at Oceti Sakowin Camp on the edge of the Standing Rock Sioux Reservation on December 4, 2016 outside Cannon Ball, North Dakota. The US Army Corps of Engineers announced today that it will not grant an easement to the Dakota Access Pipeline to cross under a lake on the Sioux Tribes Standing Rock reservation, ending a months-long standoff. (Photo by Scott Olson/Getty Images) Getty Images Sioux from Standing Rock claim victory over Dakota Pipeline Activists celebrate at Oceti Sakowin Camp on the edge of the Standing Rock Sioux Reservation on December 4, 2016 outside Cannon Ball, North Dakota, after hearing that the Army Corps of Engineers has denied the current route for the Dakota Access pipeline. The US Army Corps of Engineers on Sunday announced they will no longer allow the Dakota Access Pipeline to cross under a lake on the Standing Rock reservation in North Dakota, marking a huge win for Native Americans and protesters who had long opposed the construction. / AFP / JIM WATSON (Photo credit should read JIM WATSON/AFP/Getty Images) AFP/Getty Images

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.