US presidential candidate Hillary Clinton says she opposes the Keystone XL oil pipeline designed to carry Canadian oil to refineries on the Gulf of Mexico.
A decision on whether or not to grant a permit for the construction of the pipeline has been pending for seven years, but current president Barack Obama is expected to decide on Keystone in the coming months.
Clinton, who is running to become the Democrats' nominee for the presidency, has long avoided a firm position on the project, but environmental activists close to her campaign said she wanted to make clear her opposition before the October 13 Democratic debate, where she will face rival candidate Senator Bernie Sanders, who opposes the pipeline and had urged her to take a position.
Speaking at a town hall event in Iowa, Clinton said the prolonged debate over the controversial project has become "a distraction from the important work we have to do to combat climate change", before confirming that she opposed the pipeline.
The project occupies a key fault line between two of the Democrats' largest support bases - the unions and environmentalists - and the former secretary of state has been accused of being evasive out of political expediency, though she has said she did not want to "second-guess" former boss Obama.
The State Department has said the pipeline would directly create 3,900 construction jobs for the year or two it was being built, plus 35 full-time jobs and 15 temporary contractor jobs once completed, and many labour unions have pushed for approval.
However, environmentalists say the project risks delaying the transition to cleaner forms of energy and increasing emissions of greenhouse gases by speeding development of Canada's oil sands.
In a statement, Sanders welcomed Clinton's opposition to the pipeline saying it would be "absurd to encourage the extraction and transportation of some of the dirtiest fossil fuel on the planet".
Clinton told the Des Moines Register editorial board she would release a plan in the next few days for a clean energy agreement among the US, Canada and Mexico that she says will create jobs.
TransCanada spokesman Davis Sheremata said the company remains focused on securing a permit for the project and Canada's conservative Prime Minister Stephen Harper, a strong proponent of the pipeline, said through a spokesman that Canada knows "the American people support the project."
The White House declined to comment on Clinton's position. The State Department said there is no timeline for the completion of a review of the project before it delivers its recommendations to Obama.
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