Chairman of the Joint Chiefs of Staff Mark Milley told General Li Zuocheng of the People's Liberation Army that the United States would not strike.

One call took place on October 30, 2020, four days before the election that defeated Trump. The second call was on January 8, 2021, just two days after the insurrection at the US Capitol by supporters of the outgoing chief executive.

Milley went so far as to promise Li that he would warn his counterpart in the event of a US attack, according to the book Peril, written by Washington Post journalists Bob Woodward and Robert Costa.

"General Li, I want to assure you that the American government is stable and everything is going to be okay," Milley told him in the first call, according to the book.

"We are not going to attack or conduct any kinetic operations against you."

"If we're going to attack, I'm going to call you ahead of time. It's not going to be a surprise," Milley reportedly said.

The Associated Press obtained a copy of the book. Details from the book, which is set to be released next week, were first reported by The Washington Post on Tuesday.

The second call was meant to placate Chinese fears about the events of January 6. But the book reports that Li wasn't as easily assuaged, even after Milley promised him: "We are 100 per cent steady. Everything's fine. But democracy can be sloppy sometimes."

Milley believed the president suffered a mental decline after the election, agreeing with a view shared by House Speaker Nancy Pelosi in a phone call they had January 8, according to officials.

Milley was appointed by Trump in 2018 and later drew the president's wrath when he expressed regret for participating in a June 2020 photo op with Trump after federal law enforcement cleared a park near the White House of peaceful protesters so Trump could stand at a nearby damaged church.

Requests for comment from Milley were not immediately returned. Milley's second warning to Beijing came after Trump had fired Secretary of Defence Mike Esper and filled several top positions with interim office holders loyal to him.

The book also offers new insights into Trump's efforts to hold on to power despite losing the election to Democrat Biden.

Trump refused to concede and offered false claims that the election had been stolen. He repeatedly pressed his vice president, Mike Pence, to refuse to certify the election results at the Capitol on January 6, the event that was later interrupted by the mob.

Pence, the book writes, called Dan Quayle, a former vice president and fellow Indiana Republican, to see if there was any way he could acquiesce to Trump's request. Quayle said absolutely not.

"Mike, you have no flexibility on this. None. Zero. Forget it. Put it away," Quayle said, according to the book.

Pence ultimately agreed. He defied Trump to affirm Joe Biden's victory.

Trump was not pleased.

"I don't want to be your friend anymore if you don't do this," Trump replied, according to the book, later telling his vice president: "You've betrayed us. I made you. You were nothing."

Peril describes Trump's relentless efforts to convince Attorney-General William Barr that the election had been stolen. Barr is quoted as telling Trump, "The Justice Department can't take sides, as you know, between you and the other candidate."

According to the book, Barr had determined that allegations about rigged voting machines "were not panning out".

Barr also expressed disgust with Rudolph Giuliani and others insisting Trump had won, calling them a "clown car".

Trump's office had no immediate comment on the book.

Australian Associated Press