The usually unflappable Hillary Clinton was filmed snapping at a Greenpeace activist in a tense exchange over fossil fuel contributions after a campaign rally in New York on Thursday.

The activist asked Clinton if she would reject money from oil and gas interests, to which the candidate flatly denied that she had received any money from fossil fuel companies for her campaign, saying that she has only accepted dollars from individuals who work for those corporations.

Cell phone video of the exchange also showed a visibly irate Clinton accusing the Sanders campaign of spreading lies about her connections to the fossil fuel industry. "I am so sick -- I am so sick of the Sanders campaign lying about me," she said.

On Thursday, the Clinton campaign reiterated those claims in a statement claiming the secretary "has not taken a dollar from oil and gas industry PACs or corporations. The simple fact is that the Sanders campaign is misleading voters with their attacks."

But Clinton's statement itself is also misleading. Candidates' campaigns cannot receive money directly from corporations. Only super PACs, which can spend unlimited amounts on behalf of candidates and significantly boost their runs for the White House, can take in those funds directly.

It's true that neither Clinton's campaign nor the super PAC supporting her have taken contributions directly from fossil fuel companies. But they have received donations from employees and lobbyists in the industry. As VICE News reported earlier this month, individuals with significant fossil fuel interests have pumped $3.25 million, or approximately one in every 15 dollars donated to Priorities USA Action, a super PAC supporting Clinton, this election cycle. Sanders does not have a super PAC.

Both Democratic candidates have taken direct contributions from employees of fossil fuel companies this cycle. Clinton's campaign has received some $307,000 from these employees, while Bernie Sanders' campaign has taken in more than $54,000, according to the Center for Responsive Politics.

The Clinton campaign is differentiating workers and lobbyists in the fossil fuel industry from the companies themselves, arguing that these are simply individuals who have their own political motivations and operate independently from their employers.

On Thursday evening, the Sanders campaign responded to video of the exchange between Clinton and Greenpeace activist Eva Resnick-Day, highlighting the amount of contributions from fossil fuel lobbyists and bundlers to Clinton's super PAC.

"The truth is that Secretary Clinton has relied heavily on funds from lobbyists working for the oil, gas and coal industry," Sanders spokesman Michael Briggs wrote in an email.

But Clinton spokesman Nick Merrill pointed to Sanders' own contributions from fossil fuel employees and took issue with the Sanders campaign conflating lobbyists and employees with the fossil fuel industry in general.

"The money in question is from individuals who work for these companies," Merrill said in a statement. "By the same metric, Bernie Sanders has taken more than $50,000 on this campaign from individuals working for oil and gas companies. Assuming they don't believe their own candidate is bought by the fossil fuel industry, they should stop the false attacks and do what they've claimed the campaign is about: debating the issues."

Greenpeace USA, a nonpartisan environmental group, says that the role of special interests in politics is problematic. The group has followed the influx of campaign funds from fossil fuel lobbyists and employees to all of the candidates this cycle, focusing in particular on the Clinton campaign and the pro-Clinton super PAC in recent months. Greenpeace has staged protests with local activists at several Clinton rallies. On Tuesday night around 50 activists protested outside a Clinton fundraiser on New York's Upper East Side, demanding that Clinton come out to speak to them about taking fossil fuel donations.

The organization argues that individuals with significant interests in fossil fuels are still capable of influencing campaigns and candidates in a similar way to corporations through direct or indirect contributions.

"Because of nature of super PACs, we don't know what these donors are being promised or what these donors want for their cash," Jesse Coleman, a researcher for Greenpeace, told VICE News earlier this month.

The testy exchange with Greenpeace activist Resnick-Day came after a group of Sanders supporters had interrupted Clinton at the same rally at SUNY Purchase by staging a walkout. As the students left the room, Clinton said she regretted that they did not "want to hear the contrast between my experience, my plans, my vision, what I know I can get done and what my opponent is promising."

Greenpeace emphasized in a statement on Thursday that the environmental group has not endorsed any campaign in 2016 and denied any links between Resnick-Day and the Sanders campaign.

"Secretary Clinton is conflating Greenpeace with the Sanders campaign, but we are an independent organization, and our research team has assessed the contributions to all Presidential candidates," Greenpeace Democracy Campaign Director Molly Dorozenski said. "We have not and will not endorse candidates."

Last year, Greenpeace launched a campaign, which targeted fossil fuel money in the 2016 campaigns and asked Republican and Democratic candidates to sign a pledge rejecting direct and indirect donations from the sector. Sanders was the only candidate who signed the pledge.

On Thursday, Greenpeace restated its call for Clinton to sign onto the pledge. "We would welcome a statement from Clinton saying that she plans to stop taking this money going forward to prove to young people like Eva that she's listening to them, not her biggest donors," the group said in a statement.

Clinton's campaign has said that the candidate has never changed any of her positions or policy based on contributions from corporations, whether from Wall Street, the fossil fuel industry, or otherwise. Both Sanders and Clinton have pledged to reverse Citizens United, the 2010 Supreme Court ruling that allowed unlimited spending by corporations in political campaigns, if they win the presidency.