Major oil and gas companies, including ExxonMobil and ConocoPhillips, as well as a Canadian trade agency promoting construction of the Keystone XL pipeline, have handed over millions of dollars to the Clinton Foundation.

Environmentalists say the oil and gas dollars are cause for alarm, especially as they press Hillary Clinton, the presumed 2016 Democratic front-runner, to oppose the controversial pipeline. The money also serves as a reminder of the sharply competing interests that Clinton, who joined the foundation in 2013 after stepping down from her position as secretary of State, must balance as she mulls a White House run.

According to publicly available records, ExxonMobil, Anadarko Petroleum, and the Foreign Affairs, Trade, and Development Canada agency all opened up their checkbooks for the Clintons' charitable organization last year.

ExxonMobil has donated between $1 million and $5 million to the foundation, which works with individuals, nonprofits, and the private sector to advance a range of charitable causes. The Canadian foreign affairs, trade, and development agency donated between $250,000 and $500,000. Anadarko, one of the world's largest oil and gas companies, donated between $50,000 and $100,000.

The Canadian trade agency lists strengthening ties with the United States, an aim it suggests could be achieved by building the Keystone XL pipeline, as one of its top priorities, according to the agency's website. And the CEOs of ExxonMobil and Anadarko have publicly urged the president to green-light the pipeline, which would haul heavy crude from Canada to the Gulf Coast.

The Wall Street Journal first reported the Canadian foreign affairs agency's contributions to the foundation and its support for Keystone XL on Tuesday evening.

ExxonMobil waved away any suggestion that its donation would be used to buy influence. "We have a long history of supporting any number of charitable groups and organizations whose interests dovetail with our own, and that's certainly the case here," Richard Keil, an Exxon spokesman said. "There's no link to any political or policy issues."

The Canadian trade agency and Anadarko did not immediately respond to a request for comment. A spokesman for Hillary Clinton directed all questions to the foundation.

"The Clinton Foundation is a philanthropy, period," spokesman Craig Minassian said. "As with other global charities, the Clinton Foundation receives the support of individuals, organizations, and governments from all over the world because our programs are improving the lives of millions of people."

BP, ConocoPhillips, Chesapeake Energy, CITGO Petroleum, and Occidental Petroleum have also each donated between $10,000 and $25,000 to the foundation. In addition, Dow Chemical, a chemicals manufacturer that has also backed construction of Keystone, contributed to the foundation last year, donating between $1 million and $5 million.

The charitable organization has also collected cash from a wide variety of donors, including environmental groups such as the Nature Conservancy.

But the millions of dollars funneled to the Clinton foundation from oil and gas companies and Keystone supporters still don't sit well with environmentalists.

"We've long been concerned about Hillary Clinton's ties to the oil and gas industry," said Ben Schreiber, the climate and energy program director for Friends of the Earth. "It doesn't shock us to see that these companies have been giving to the foundation, but it certainly raises a red flag. We're concerned about the influence that these petrodollars have."

Friends of the Earth and dozens of other environmental groups, including Greenpeace, penned a letter to Clinton last year calling on her to take a stand against the pipeline.

But Clinton has so far kept quiet when it comes to Keystone. In January, Clinton dodged a question about her stance on the project, saying she does not want to weigh in while the Obama administration's review of the project is underway.

That stance has frustrated environmentalists, who want to see Clinton demonstrate that environmental issues would be a top priority if she were to run for president.

Clinton has so far attempted to stake out a middle ground on energy and environmental policy. She called climate change "the most consequential, urgent, sweeping collection of challenges we face as a nation and a world" at an event in Las Vegas last year. The former secretary of State has also offered praise for fracking, the technology that has enabled the nation's natural-gas boom. 

"As far as I'm concerned, there's no stronger proponent of environmental issues than former Secretary Clinton. For me this just indicates how tricky fundraising is," said Jim Manley, a former aide to Senate Minority Leader Harry Reid.

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