US President Donald Trump behaved “like a dog off the leash” at several events he attended without his wife, Melania Trump, a former White House staffer claims in an explosive memoir.

In her new book, Unhinged, former staffer Omarosa Manigault Newman also claimed there were tapes of Mr Trump using racial slurs, one of many scandalous claims in the tell-all to be released this week.

The Associated Press purchased an early copy of the memoir, which the White House has already slammed as “riddled with lies and false accusations”.

In the book, Ms Manigault Newman, who was a contestant on Mr Trump’s reality TV show, The Apprentice, and later served as a senior adviser to the President, hurls a litany of allegations, painting the picture of a scattered, self-absorbed, misogynistic and insecure wreck of a man.

And after years of defending Mr Trump, she wrote in the book that she had concluded he was in fact a bigot.

“I didn’t want to believe it,” she wrote. “I rejected what other people said about him because they didn’t know him like I did. I had to go through the pain of witnessing his racism with my own eyes, and hearing it with my own ears, many times, until I couldn’t deny it any longer.”

She also claimed, without evidence, that there are tapes of the President using the ‘N’ word repeatedly on the reality show’s set. While she acknowledged she had never been able to obtain or hear the tapes, she said three unnamed sources had described their contents to her.

Donald Trump and reality TV star and former aid Omarosa Manigault.

Ms Manigault Newman also alleges that the President’s associates tried to buy her silence.

On Friday, NBC News reported that Ms Manigault Newman also wrote that 24 hours after she left her role in the White House in late 2017, Mr Trump’s daughter-in-law, Lara Trump, emailed her a contract to work on the President’s re-election campaign for $US15,000 ($20,540) per month.

The contact came with a nondisclosure agreement that would prevent Ms Manigault Newman from “ever talking about the entire Trump family or the entire Pence family, to anyone in the universe, for all of eternity”, she reportedly wrote.

“I declined the offer. I was done with Trumpworld.”

After turning down the job, she claims she received a “flurry” of letters from attorneys representing the President, all telling her to “stay silent about Mr Trump, or else.”

Throughout the book, Ms Manigault Newman paints a deeply critical portrait of the President, describing him as a man who “loved conflict, chaos and confusion; he loved seeing people argue or fight”.

She says she had seen him acting inappropriately at numerous events he attended without his wife at his Mar-a-Lago estate in Florida, including birthday parties, fundraisers and golf tournaments.

She also alleges that Mr Trump has exhibited signs of a “mental decline that could not be denied” and says she went as far as printing out a study linking Diet Coke, the teetotal President’s favourite drink, to dementia and strokes and putting it in his briefing stack.

The White House responded by slamming the book and its author and chastising the press for writing about it.

“It’s sad that a disgruntled former White House employee is trying to profit off these false attacks, and even worse that the media would now give her a platform, after not taking her seriously when she had only positive things to say about the President during her time in the administration,” White House spokeswoman Sarah Huckabee Sanders said.

-with AAP