Donald Trump has picked a prominent climate change sceptic to help him craft his energy policy and pushed back against renewed calls that he release his income tax returns - saying his tax rate is "none of your business".
The presumptive Republican presidential nominee is seeking to build out his policy proposals as he pivots from campaigning for his party's nomination to a likely general election match-up with Democratic rival Hillary Clinton.
Among those he has asked for help is US Republican Representative Kevin Cramer of North Dakota, one of the country's most ardent oil and gas drilling advocates and climate change sceptics. North Dakota has been at the forefront of the US shale oil and gas boom.
Trump's team asked Cramer, who has endorsed Trump, to write a white paper, or detailed report, on his energy policy ideas, according to Cramer and sources familiar with the matter.
Cramer has said his white paper will emphasise the dangers of foreign ownership of US energy assets, as well as what he characterises as burdensome taxes and over-regulation. Trump would have an opportunity to float some of the ideas at an energy summit in Bismarck, North Dakota, on May 26, Cramer said.
Environmental groups and Clinton's campaign quickly attacked Trump for tapping Cramer.
"Kevin Cramer has consistently backed reckless and dangerous schemes to put the profits of fossil fuel executives before the health of the public, so he and Trump are a match made in polluter heaven," Sierra Club legislative director Melinda Pierce said in an emailed statement.
Clinton campaign spokesman Jesse Ferguson said, "Donald Trump's choice of outspoken climate (change) denier Kevin Cramer to advise him on energy policy is just the latest piece of evidence that letting him get near the White House would put our children's health and futures at risk."
Trump recently told supporters in West Virginia the coal industry would thrive if he were president. He has also claimed global warming is a concept "created by and for the Chinese" to hurt US business.
Clinton, meanwhile, has advocated shifting the country to 50 per cent clean energy by 2030, promised heavy regulation of fracking, and said her prospective administration would put coal companies "out of business".
Trump also took heat on Friday for not releasing his tax returns, something American presidential candidates have done for decades. Clinton and her rival, Senator Bernie Sanders of Vermont, have both released their returns.
Trump has said the Internal Revenue Service was auditing his returns and he wanted to wait until the review was over before making them public.
Pressed on what tax rate he paid, Trump refused to say.
"It's none of your business," he said.
The IRS declined to comment on whether any presidential candidates were being audited.
