The new head of the US Environmental Protection Agency (EPA), Scott Pruitt, says he does not agree CO2 is a primary contributor to global warming.

In an interview with CNBC, Mr Pruitt also said the Government would make an announcement on fuel-efficiency standards for cars "very soon", stressing he and President Donald Trump believed current standards were rushed through.

Mr Pruitt, 48, is a climate change denier who sued the agency he now leads more than a dozen times as Oklahoma's attorney-general. He said he was not convinced carbon dioxide pollution from burning fossil fuels like oil, gas and coal was the main cause of climate change, a conclusion widely embraced by scientists.

"I think that measuring, with precision, human activity on the climate is something very challenging to do and there is tremendous disagreement about the degree of impact," he said.

"So no, I would not agree that it's a primary contributor to the global warming that we see.

"But we don't know that yet, we need to continue to debate, continue the review and analysis."

Trump campaigned on a promise to roll back environmental regulations ushered in by former president Barack Obama, including those aimed at combating climate change.

He framed his stand as aimed at boosting US businesses, including the oil and gas drilling and coal mining industries.

Scientists immediately criticised Mr Pruitt's statement, saying it ignored a large body of evidence collected over decades that showed fossil fuel burning was the main factor in climate change.

"We can't afford to reject this clear and compelling scientific evidence when we make public policy. Embracing ignorance is not an option," Ben Santer, climate researcher at Lawrence Livermore National Laboratory, said in a statement.

'The mask is off'

The Supreme Court unleashed a fury of regulation and litigation when it ruled in 2007 that greenhouse gases were an air pollutant that could be regulated under the Clean Air Act. Two years later, the EPA declared carbon dioxide and five other heat-trapping gases to be pollutants.

"Nowhere in the continuum, nowhere in the equation, has Congress spoken. The legislative branch has not addressed this issue at all," Mr Pruitt said.

US President Donald Trump believes the Paris Agreement was a "bad deal". ( Reuters: Jonathan Ernst )

He said the Supreme Court's decision should not have been viewed as permission for the EPA to regulate carbon dioxide emissions.

"So I think all those things need to be addressed as we go forward, not least of which is the response of the legislative branch with respect to this issue," he said.

As Oklahoma's attorney-general, Mr Pruitt and another dozen attorneys-general unsuccessfully challenged the endangerment finding in a federal appeals court.

"The mask is off. After obscuring his true views during his Senate confirmation hearings, Scott Pruitt has outed himself as a pure climate denier," said David Doniger, director of the climate program at the Natural Resources Defence Council.

The new EPA chief said he was committed to ensuring thorough processes for environmental rules and regulations to reduce "regulatory uncertainty".

Mr Pruitt added that he shared Mr Trump's view the global climate accord agreed by nearly 200 countries in Paris in 2015 was a "bad deal". Mr Trump promised during his campaign for the White House to pull the United States out of the accord, but has since been mostly quiet on the issue.

ABC/Reuters