US presidential nominee Hillary Clinton on Monday went after her opponent Donald Trump over his possible use of deductions from business losses in the 1990s to avoid paying income taxes for several years.

"Trump represents the same rigged system that he claims he's going to change," Clinton said at a campaign event in Ohio. "The whole story tells us everything we need to know about how Trump does business."

She called for efforts to make sure US businesses engage in fair practices.

Trump is "in a category by himself," she said two days after the New York Times broke the story about a 1995 income tax document it obtained and the tax benefits the Republican presidential nominee possibly took advantage of.

Tax experts hired by the Times and who analyzed the three-page document said that tax rules especially advantageous to wealthy people would have allowed Trump legally to use an 916-million-dollar loss to cancel out an equivalent amount of taxable income over an 18-year period.

Clinton zeroed in on both the size of the loss and the tactic Trump used.

"What kind of genius loses a billion dollars in a single year?" Clinton said.

"He's taken corporate excess and made a business model out of it. He abuses his power, games the system, puts his own interests ahead of the country's," she said. "It's Trump first and everyone else last."

The story said the tax documents showed the loss came from the financial mismanagement of casinos in Atlantic City, New Jersey, his attempt to start an airline and his purchase of a large hotel in Manhattan.

Trump, speaking at a campaign rally in Colorado, said he had a responsibility to pay as little tax as legally possible and noted Clinton did not have business experience.

"I have legally used the tax laws to my benefit and to the benefit of my company and my investors and my employees," he said.

Trump also pledged to "straighten out" the tax system, saying "I understand the tax laws better than almost anyone and that is why I am the one who can fix them."

"The unfairness of the tax law is unbelievable," Trump said. "It's something that I have been talking about for a long time despite being a very big beneficiary [of the tax law]."

On Sunday his campaign issued a statement that neither challenged nor confirmed the authenticity of the tax document. It defended Trump for paying hundreds of millions of dollars in other taxes, including property taxes and sales and excise taxes.

Trump also on Monday faced more controversy over his charitable foundation. The attorney general of New York said the private foundation violated New York laws by not properly registering as a charity with the state.

The state ordered the foundation to stop soliciting contributions because it was doing so without having properly registered with the state's Charities Bureau to accept contributions or providing annual financial statements.

Trump's charity has come under media scrutiny, with questions raised about his personal contributions and whether he improperly benefited from its charitable work.

The Washington Post has examined Trump's charitable giving and noted no personal contributions to the foundation from Trump since 2008. Trump has relied instead on other donors to keep his foundation's coffers filled, which requires more comprehensive reporting to New York state authorities.

The clash over Trump's taxes came as Clinton gained a five-point lead over Trump after last week's first presidential debate, according to an opinion poll published Monday.

Clinton led Trump by 47 per cent to 42 per cent among likely voters, the CNN/ORC poll said, with most of her gains apparently coming from men and independents.

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