US has devolved into the 'incredibly dysfunctional family' of Donald Trump, says Mary Trump

Donald Trump's niece has said in a new interview that the leadership of the US has devolved into a macro version of her "incredibly dysfunctional family", which she blames on the president's father.

To promote her upcoming book, Mary Trump said in a series of interviews that the family patriarch Fred Trump is "almost 100 per cent" responsible for creating the "killer" elected to the White House.

Ms Trump told The Washington Post that her extended family's dynamic is playing out on the national stage, with the president displaying "an unerring instinct for finding people who are weaker than he is".

"[Mr Trump is] eminently usable by people who are stronger and savvier than he is," Ms Trump told the Post.

Too Much and Never Enough: How My Family Created the World's Most Dangerous Man was published on Tuesday an became a best-seller after a New York Supreme Court judge ruled on Monday that Ms Trump was free to release and talk about the president's private life.

The president everyone knows today all began at the hands of his father, who was a sociopath who destroyed his family with no empathy, Ms Trump said in an interview with ABC's George Stephanopoulos.

"If somebody could be of service to him [Fred Trump], then he would use them. If they couldn't be, he excised them," Ms Trump said.

She continued: "And Donald learned that lesson. And he essentially had to sacrifice whatever goodness there may have been in him once, whatever capacities for experiencing the full range of human emotion, to my grandfather."

The lesson Mr Trump learned from his father, Ms Trump said, was to become a "killer".

"The man who needs to succeed at all costs, who recognises that other people are expendable, who does not need to take responsibility, who will do anything to get attention, financial rewards, and to 'win'," she said.

While Mr Trump has not commented on the latest interviews, White House press secretary Kayleigh McEnany told the Post that Ms Trump's tome is a "book of falsehoods, plain and simple".

The 214-page memoir is the first book written by a Trump family insider with an intimate knowledge of the president's upbringing and private life.

The book opens with Ms Trump's account of long being estranged from the family after a dispute over the inheritance of her grandfather's estate in 1999.

She visited her uncle in the White House a few months after inauguration, and when asked what she would say if she visited him in the Oval Office again, she replied "resign".

"Donald's pathologies are so complex and his behaviours so often inexplicable that coming up with an accurate and comprehensive diagnosis would require a full battery of psychological and neuropsychological tests that he'll never sit for," she writes in the book.

"At this point, we can't evaluate his day-to-day functioning because he is, in the West Wing, essentially institutionalised. Donald has been institutionalised for most of his adult life, so there is no way to know how he would thrive, or even survive, on his own in the real world."