US president Donald Trump's older sister branded him as cruel and a liar whose lack of principles meant he could not be trusted. This is the outcome of a secretly recorded audio published Saturday.

This time the accusations about the US president came from one of his own. Maryanne Trump Barry, 83, a former federal judge, said her brother "doesn't read", and slammed him as "cruel" over his immigration policies which saw thousands of children separated from their families and held in detention centres.

The conversations with Barry were secretly recorded by her now estranged niece Mary Trump. It brings out a deep lack of trust in the president from within his own family. It is also a potentially damaging blow ahead of the US presidential elections on Nov. 3.

"All he wants to do is appeal to his base," she said in recordings obtained by the Washington Post as reported by Daily Mail. "He has no principles. None. His goddamned tweet and lying, oh my God," she said.

Mary's bombshell book "Too Much and Never Enough: How My Family Created the World's Most Dangerous Man," published in July was about the "toxic family" that produced him. But, the source of her allegations were never revealed previously.

The president's younger brother Robert Trump, 71, who died on Aug. 15, went to court to try to block its publication. He argued that Mary was violating a non-disclosure agreement signed in 2001 after the settlement of her grandfather's estate, but to no avail.

Around 950,000 copies of the book were sold the day of its release, with the White House calling it a "book of falsehoods".

Barry said to her niece: "It's the phoniness of it all. It's the phoniness and this cruelty. Donald is cruel."

The recordings also shed light on an embarrassing claim in the memoir that the president paid someone to take a college entrance exam for him. "He got into University of Pennsylvania because he had somebody take the exams," Barry said, adding she even remembered the man's name.

President Donald Trump
President Donald Trump will be a prime target of speeches at the Democrats' national convention starting August 17 Photo: AFP / JIM WATSON

The White House or the president did not immediately comment on the recordings.