The US presidential candidates traded personal attacks on Tuesday as they provided more access to reporters trying to keep up with the quickening pace of the campaign.

Republican Donald Trump, in another provocative comment about a woman's appearance, said Democratic candidate Hillary Clinton lacks "a presidential look," while she criticized him for not releasing his tax returns, saying he "clearly has something to hide."

"I just don’t think she has a presidential look, and you need a presidential look,” Trump told ABC News.

Trump has already been criticized as being sexist for comments he made during the state-by-state primaries about female journalists, entertainers and the only female candidate on the Republican side, Carly Fiorina.

The latest comment is not likely to help him close the gender gap that has appeared in a number of polls of likely female voters. A recent one, conducted Thursday through Sunday by CNN/ORC, shows that Clinton leads Trump 52 per cent to 38 per cent among likely female voters.

Christina Reynolds, the Clinton campaign's deputy communications director, said the comment wasn't the first time Trump had made such a statement.

"So it’s not surprising that Donald Trump doesn’t think Hillary Clinton looks presidential," Reynolds said, according to the New York Times. "Voters know all too well what’s not presidential: Donald Trump and his narrow views and divisive rhetoric."

Trump fired back, saying that Clinton had said things about him that were "horrible" and had questioned his temperament, which he said was "the single greatest asset I have."

On the subject of Trump's federal income tax returns, Clinton, who for the second day in a row took questions from reporters flying with her on her campaign plane, said Trump is "dead wrong" that Americans don't care about seeing them before the election in November.

"The burden is on him, and indeed for the rest of us in the campaign on our side, the press, the public, to demand what a big majority of the public says they want," Clinton said.

The former US secretary of state also blasted her opponent over Trump University.

Clinton referenced a report in the Washington Post which said that Trump contributed 25,000 dollars to the campaign of Florida Attorney General Pam Bondi in 2013, when she was deciding whether to investigate allegations of fraud against the university, which she ultimately opted not to do.

Former enrollees of the school, which offered a real estate training programme, have sued it for using misleading marketing tactics.

The Democratic nominee said this was another example of the scams, frauds and questionable relationships and business activities that swirl around Trump.

Trump told reporters on his plane that he never spoke to Bondi, adding, "Many of the attorney generals turned that case down because I'll win that case in court."

At a campaign stop in Greenville, North Carolina, Trump repeated criticism of the way Clinton handled her email while she was secretary of state and accused her of using the State Department to dole out special favours and access to her friends and donors to the Clinton Foundation.

Clinton, who was also interviewed by ABC, said she was proud of the Clinton Foundation's work, adding, "No decision I ever made [as secretary of state] was influenced by anybody."

Her decisions while in office were based on "what was good for the United States, what was good for our values, our interests and our security, and as the State Department has confirmed, there is no evidence of any such influence at all."

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