Bolton, who acrimoniously left the White House a day before Trump ultimately released the Ukraine aid on September 11, has already told lawmakers he is willing to testify if subpoenaed, despite the President's order barring aides from co-operating in the trial.

"I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens. In fact, he never complained about this at the time of his very public termination. If John Bolton said this, it was only to sell a book," Trump tweeted.

The White House did not respond to a request for comment on Sunday night, US time. Trump's personal lawyer Rudy Giuliani, who has played a prominent role in the Ukraine affair, replied to a request for comment with a text: "I used to like and respect John and tell people they were wrong about how irresponsible he was. I was wrong."

Democrats need at least four Republicans to vote with them to seek witness testimony. Those prospects looked unlikely in recent days and it's unclear if the new revelations about Bolton's book will sway any Republican senators.

Democrats quickly sought to ramp up the pressure on their Republican counterparts calling for Bolton to testify.

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"John Bolton has the evidence," tweeted Senate Minority Leader Chuck Schumer. "It's up to four Senate Republicans to ensure that John Bolton, Mick Mulvaney, and the others with direct knowledge of President Trump's actions testify in the Senate trial."

Two people familiar with the book, who requested anonymity because of the sensitivity of the project, confirmed to The Washington Post that it details Trump tying aid to the desire for Biden probes and details a number of conversations about Ukraine that he had with Trump and key advisers, such as Secretary of State Mike Pompeo. They said Bolton was ready to testify.

In a joint statement, the seven House impeachment managers - as the Congress members acting as prosecutors are known - called the report "explosive" and urged the Senate, controlled by Republicans, to agree to call Bolton as a witness.

"The Senate trial must seek the full truth and Mr Bolton has vital information to provide," the managers said in the statement. "There is no defensible reason to wait until his book is published, when the information he has to offer is critical to the most important decision senators must now make - whether to convict the president of impeachable offences."

The Associated Press has not confirmed the content of Bolton's draft book. A person familiar with the matter said the book had been submitted to the White House for pre-publication review, which is standard for the work of former officials with security clearances. The person insisted on anonymity to discuss the sensitive subject.

The book's publisher, Simon & Schuster, declined to comment.

Sarah Tinsley, an adviser to Bolton, said: "The ambassador's manuscript was transmitted to the White House in hard copy several weeks ago for pre-publication review by the NSC. The ambassador has not passed the draft manuscript to anyone else. Period."

Bolton lawyer Charles Cooper said in a statement the pre-publication review process had been "corrupted and that information has been disclosed by persons other than those properly involved in reviewing the manuscript."

Democrats accuse Trump of abuse of power in withholding the military assistance to Ukraine to push that country to mount investigations into Democrats, including former vice-president Joe Biden and his son Hunter, who served on the board of a Ukraine gas company, Burisma, while his father was in office.

The President's lawyers said during their opening day of defence arguments at the weekend that there was no evidence  Trump made the military aid contingent on the country announcing an investigation into Biden.

The New York Times also reported that Bolton said he told Attorney-General William Barr that he was mentioned by Trump on his July 25 call with Ukrainian President Volodymyr Zelensky. A Justice Department official on Sunday disputed Bolton's account that he had told Barr about the President's call with Zelensky but did say that Bolton had called Barr to raise concerns about Giuliani's shadow diplomacy efforts.

Trump on Wednesday told reporters in Davos, Switzerland, that he didn't want Bolton to testify before the Senate.

"The problem with John is it's a national security problem," Trump said. "He knows some of my thoughts. He knows what I think about leaders. What happens if he reveals what I think about a certain leader and it's not very positive and then I have to deal on behalf of the country?"

He added: "It's going to be very hard. It's going to make the job very hard."

AP, agencies