Republican presidential candidate Donald Trump on Thursday denied reports that he sexually assaulted multiple women, attacking the media for spreading "vicious claims" and "outright lies."

"The slander and libel that was just last night thrown at me by the Clinton machine and The New York Times and other media outlets was part of a concerted, coordinated and vicious attack," Trump said at a campaign rally in Florida.

The New York Times reported late Wednesday that two women claimed Trump had groped and kissed them without their consent, and People Magazine later said one of its reporters had also been assaulted by Trump during an interview.

Jessica Leeds, 74, told the Times that Trump had grabbed her breasts and tried to put his hand up her skirt during a flight to New York more than three decades ago. Rachel Crooks told the paper she encountered Trump outside an elevator in 2005 and that he kissed her without permission.

Neither reported the incidents to authorities and never before made their stories public, according to the Times.

Trump charged the media were working to help rival Hillary Clinton win the election and called the media "the most powerful weapon deployed by the Clintons."

"Their agenda is to elect crooked Hillary Clinton at any cost, at any cost no matter how many lives they destroy," he said.

Clinton did not address the allegations directly, but told campaign volunteers in San Francisco that the US could not let Trump's "pessimistic vision" take hold and repeated remarks by First Lady Michelle Obama calling for Democrats to take the high road.

"When they go low, we go high," she said.

Michelle Obama issued a forceful denunciation of Trump at a campaign rally for Clinton in New Hampshire, without uttering the Republican's name.

He had "said things about women that are so shocking, so demeaning that I simply will not repeat anything here today," she said, noting the remarks had "shaken me to my core."

"This is not something that we can ignore," Obama said. "This wasn't just locker-room banter. This was a powerful individual speaking freely and openly about sexually predatory behavior, and ... using language so obscene that many of us were worried about our children hearing it when we turn on the TV."

Trump said the Times article, along with an earlier report by the newspaper on Trump's treatment of a female employee, would be part of a lawsuit against the newspaper.

The newspaper declined a request from Trump's lawyer to remove the latest article from its website.

The report did not meet the lawyer's claim of being libelous because it did not damage Trump's reputation amid his own past sexually explicit remarks, the Times said.

"Nothing in our article has the slightest effect on the reputation that Mr Trump through his own words and actions, has already created for himself," Times lawyer David McCraw wrote in a letter to Trump's lawyer.

The attacks on the press are the latest by Trump, who routinely scorns the media's treatment of his campaign and has banned certain reporters from his rallies.

The Committee to Protect Journalists warned in a statement Thursday that Trump posed a threat to freedom of the press.

Trump "would represent a threat to press freedom in the United States, but the consequences for the rights of journalists around the world could be far more serious," CPJ chair Sandra Mims Rowe said.

Disregard of a free press in the US could embolden "dictators and despots" elsewhere, Rowe said.

The allegations in the New York Times report come almost a week after a 2005 tape was leaked to media in which Trump discusses using his celebrity status to sexually assault women.

During Sunday's presidential debate, Trump emphatically denied that he had ever kissed or groped women without permission.

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Report: Two women experienced Trump's unwanted advances

New accusers say Trump sexually assaulted them

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