US energy giant ExxonMobil is facing an onslaught from environmentalists and some shareholders alleging it hid what it knew about the effects of fossil fuels on climate change.

In an ironic twist: among the opponents is the Rockefeller Family Fund, built on the fortune amassed by John D. Rockefeller, founder of Standard Oil, which became Esso, then Exxon and then, in 1999, ExxonMobil.

The RFF met last January, in secret, in Manhattan with environmental nongovernmental groups "to establish in the public's mind that Exxon is a corrupt institution that has pushed humanity (and all creation) towards climate chaos and grave harm," according to an internal document on the meeting seen by AFP.

"We hosted a meeting with leading advocates to understand their thoughts on how to best respond to the outrageous conduct," Lee Wasserman, the director of RFF, told AFP.

They adopted a strategy to attack ExxonMobil on legal grounds, by convincing authorities to launch investigations and by filing lawsuits. In other words, replicating the tactics used against the tobacco industry in the 1990s.

"This is a conspiracy to deliberately misrepresent the company position and to tear down the company," Alan Jeffers, a spokesman for ExxonMobil, told AFP.

According to a person close to the situation who requested anonymity, certain members of the Rockefeller family have privately expressed opposition to the campaign against ExxonMobil.

Bill McKibben, founder of the NGO 350.org who participated in the January meeting, has pushed for investigations into whether ExxonMobil broke the law.

"We want everyone we can think of to know it broke every kind of moral law," McKibben said in an email.

The ecologist led the ultimately successful opposition against the Keystone XL pipeline, which would have brought Canadian oil sands production from Alberta to the US Gulf states.

The ExxonMobil critics accuse the oil company of having, since 1977, research showing that fossil-fuel energy has a harmful impact on climate, but that it kept the information to itself.

Denouncing the critics charges as "inaccurate" and a "conspiracy", ExxonMobil insists that it had acknowledged the risks of climate change as soon as it was possible, that is, in the 2000s.

Accusing the Rockefeller organization of influencing the media and the authorities, the Texas firm has pledged to publicly defend its positions, although until now it has maintained a certain discretion about the subject.

The change in attitude is due to the power of the Rockefellers, who not only have the colossal financial means to contest ExxonMobil on all battlegrounds, but also the influence of their powerful family name.

- InsideClimate, Columbia probes -

The RFF has based its opposition on two separate investigations, by the InsideClimate News and the journalism school at Columbia University in New York, that found ExxonMobil knew in the 1970s that fossil fuels were a major source of climate change.

InsideClimate, like the Columbia school, received financing from the Rockefellers.

The RFF also criticizes ExxonMobil's support of think tanks "which helped create doubt about the profound risks associated with climate change."

That includes the lobbying group the American Legislative Exchange Council, which is financed by companies including ExxonMobil, Jeffers acknowledges. Cynthia Bergman, an executive at the oil group, is a member of ALEC's advisory council.

Since the probe revelations, a number of states have launched investigations into whether it lied about climate change. The state of New York, for example, has requested documents related to the allegations and a list of associations and NGOs financed by the company.

While Jeffers says the company is cooperating with these investigations, ExxonMobil is fighting a legal battle over an investigation launched by the US Virgin Islands as the group of Caribbean islands faces the threat of rising water levels blamed on climate change.

The activists and the RFF could score a first victory on May 25 if ExxonMobil shareholders approve a resolution requiring the company to disclose the impact of climate change on its business, a policy agreed at the climate summit in Paris last year.

A dozen shareholders, including the huge California state pension fund CalPERS, French bank BNP Paribas and insurer AXA, are backing approval of the measure.

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