Donald Trump has said he supports a controversial oil pipeline that runs next to a Native American reservation in North Dakota – a project that the president-elect is personally invested in.

A briefing from Trump’s transition team said that the real estate magnate supports the construction of the Dakota Access pipeline and that his backing “has nothing to do with his personal investments and everything to do with promoting policies that benefit all Americans”.

Financial disclosure forms released earlier this year show that Trump has a stake in Energy Transfer Partners, the Texas-based firm behind the pipeline, and Phillips 66, which will hold a share of the project once completed.

Trump’s investment in Energy Transfer Partners dropped from between $500,000 and $1m in 2015 to between $1,500 and $50,000 this year. His stake in Phillips 66, however, rose from between $50,000 and $100,000 last year to between $250,000 and $500,000 this year, according to the forms.

The financial relationship has run both ways. Kelcy Warren, chief executive of Energy Transfer Partners, gave $103,000 to elect Trump and handed over a further $66,800 to the Republican National Committee after the property developer secured the GOP’s presidential nomination.

However, Trump’s transition team dismissed any conflict of interest. “Those making such a claim are only attempting to distract from the fact that president-elect Trump has put forth serious policy proposals he plans to set in motion on day one,” said a briefing note that was sent to campaign supporters.

In this 25 November 2016 satellite image taken by DigitalGlobe, construction of the Dakota Access pipeline is shown at the top right. Photograph: AP

Mary Sweeters, a spokesperson for Greenpeace, said Trump’s support showed that “crony capitalism will run his administration”.

“This is the definition of corruption,” she said. “The president of the United States should not be trading favors with oil and gas corporations. Millions of people will lose access to a clean water supply, including the Standing Rock Sioux tribe, and the rest of America will face the impacts of catastrophic climate change from burning fossil fuels.”

A protest camp has grown in North Dakota since April, amid fears that the $3.8bn Dakota Access pipeline will threaten the water and cultural artifacts of the Standing Rock Sioux tribe. The 1,170-mile pipeline will take oil from North Dakota’s Bakken fields to a refinery in Illinois and will cross the Missouri river – the main source of water for the tribe.

The long-running protest has unified Native American tribes against the project, with repeated clashes between protesters and police. This week, North Dakota governor Jack Dalrymple ordered the immediate evacuation of the protest camp, amid accusations of police violence from the mass arrests and water cannon deployment that have echoes of the civil rights protests of the 1960s.

Barack Obama has suggested that the pipeline be rerouted to allay the fears of tribes but Energy Transfer Partners has vowed to push ahead and has accused the federal government of imposing costly delays upon the project.

Trump has promised an “America first” energy policy that will attempt to boost domestic oil, coal and gas production. The president-elect has already stated he wants to “lift the Obama-Clinton roadblocks and allow vital energy infrastructure projects to move forward”, including the Keystone pipeline, another controversial fossil fuel project.

North Dakota senator John Hoeven, a Republican, said that he met with Trump and successfully urged him to support the Dakota Access pipeline.

“Mr Trump expressed his support for the Dakota Access pipeline, which has met or exceeded all environmental standards set forth by four states and the Army Corps of Engineers,” Hoeven said.

“Also, it is important to know that the new administration will work to help us grow and diversify our energy economy and build the energy infrastructure necessary to move it from where it is produced to where it is needed. The result will be more jobs, a more vibrant economy and affordable energy for the American people.”