Donald Trump on Friday 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 flesh out his policy proposals as he pivots from campaigning for his party's nomination to a probable general election matchup 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 said in an interview that his white paper would emphasise the dangers of foreign ownership of US energy assets, as well as what he characterised as burdensome taxes and over-regulation. Trump will have an opportunity to float some of the ideas at an energy summit in Bismarck, North Dakota on May 26, Cramer said.

The senator was also among a group of Trump advisers who recently met with lawmakers from Western energy states who hope Trump will open more federal land for drilling, a lawmaker who took part in the meeting said.

A spokeswoman for Trump's campaign did not comment.

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.

The Clinton campaign also criticised the move.

"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," said campaign spokesman Jesse Ferguson.

Trump has been light on the details of his energy policy, though he recently told supporters in West Virginia that 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".

"None of your business"

Trump also took heat on Friday for not releasing his tax returns, something that 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. "It should be, and I hope it's before the election," Trump told ABC's Good Morning America.

Pressed on what tax rate he pays, Trump refused to say. "It's none of your business," he said. The candidate has said there is nothing voters can learn from his tax filings.

Tax filings show sources of income, both from within the United States and other countries, as well as charitable giving, investments, deductions and other financial information.

The IRS declined to comment on whether Trump or any other presidential candidates were being audited.

However, the Trump campaign earlier this year released a letter from his attorneys saying his personal tax returns have been under "continuous examination" from the IRS.

This week, Clinton began calling on her probable Republican rival to release his returns. Last August, the former US secretary of state posted the past eight years of tax returns for her and her husband, former President Bill Clinton, on her website. Sanders released his 2014 return in April.

Presidential candidates have a long history in the modern era of releasing their tax returns.

"In 1976, Gerald Ford did not release his returns, but he did release some information about his taxes," said Joseph Thorndike, director of the Tax History Project at Tax Analysts, a nonprofit, nonpartisan group that provides tax news and analysis.

"That was the last time that a major party nominee hasn't done it," he said.