Trump’s 2016 federal disclosure forms show he owns stock in the company building the disputed oil pipeline.

President-elect Donald Trump holds stock in the company building the disputed Dakota Access oil pipeline, and opponents of the project warn those investments could affect any decision he makes on the $3.8bn project as president.

Trump’s 2016 federal disclosure forms show he owned between $15,000 and $50,000 in stock in Texas-based Energy Transfer Partners.

That’s down from between $500,000 and $1m a year earlier.

Trump also owns between $100,000 and $250,000 in Phillips 66, which has a one-quarter share of Dakota Access.

Opinion: N Dakota pipeline protest is a harbinger of many more

While Trump’s stake in the pipeline company is modest compared with his other assets, ethics experts say it is among dozens of potential conflicts that could be resolved by placing his investments in a blind trust, a step Trump has resisted.

“Trump’s investments in the pipeline business threaten to undercut faith in this process – which was already frayed – by interjecting his own financial wellbeing into a much bigger decision,” Sharon Buccino, director of environment group Natural Resources Defense Council, told the Associated Press news agency.

Concern about Trump’s possible conflicts comes as protests over the pipeline have intensified in recent weeks, with total arrests since August rising to 528.

A clash this past week near the main protest camp in North Dakota left a police officer and several protesters injured.

In an interview with Al Jazeera, Orlando Cruz, a Native American protester, said the pipeline symbolises the subjugation of his people by the government for centuries.

“They took our land from us. They said, ‘Here, this is yours, here’s a reservation, you can do what you want on it.’ And we are here in the reservation now, and we don’t get to do what we want with it. And they get to put their pipeline through it,” he said.

‘Pay-to-play at its rawest’

The Obama administration said this month that it wants more consideration and tribal input before deciding whether to allow the partially built pipeline to cross under a Missouri River reservoir in North Dakota.

The 1,200-mile pipeline would carry oil across four states to a shipping point in Illinois.

The project has been held up while the Army Corps of Engineers consults with the Standing Rock Sioux, who believe the project could harm the tribe’s drinking water and Native American cultural sites.

The delay raises the likelihood that a final decision will be made by Trump, a pipeline supporter who has vowed to “unleash” unfettered production of oil and gas. He takes office in January.

Trump, a billionaire who has never held public office, holds ownership stakes in more than 500 companies worldwide.

He has said that he plans to transfer control of his company to three of his adult children, but ethics experts have said conflicts could engulf the new administration if Trump does not liquidate his business holdings.

Arizona Congressman Raul Grijalva, the senior Democrat on the House Natural Resources Committee, called Trump’s investment in the pipeline company “disturbing” and said it fits a pattern evident in Trump’s transition team.

“You have climate [change] deniers, industry lobbyists and energy conglomerates involved in that process,” Grijalva said. “The pipeline companies are gleeful. This is pay-to-play at its rawest.”

Besides Trump, at least two possible candidates for energy secretary in his incoming administration also could benefit financially from the pipeline.