US President Donald Trump told a then-top aide in August he wanted to freeze security aid to Ukraine until officials there helped with investigations into Democrats, including former Vice President Joe Biden, the New York Times reported on Sunday.

Trump's statement was described in an unpublished manuscript by former NSA John Bolton, the Times said in a report that could raise pressure on Republicans to call Bolton as a witness in Trump's Senate impeachment trial.

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The report, which did not quote the manuscript but cited multiple people describing Bolton's account, may undercut a key element of Trump's defence: that there was no quid pro quo when he asked Ukrainian President Volodymyr Zelenskyy to investigate Biden and his son Hunter Biden in a July phone call.

Biden is a leading contender for the Democratic nomination to oppose Trump in the November 3 presidential election. His son worked for a Ukrainian energy firm while Biden was vice president.

In a statement, a lawyer for Bolton suggested that the Times' account was accurate and said he had submitted Bolton's book manuscript to the National Security Council on December 30, a standard security review for classified information.

"It is clear, regrettably, from The New York Times article published today that the prepublication review process has been corrupted and that information has been disclosed by persons other than those properly involved in reviewing the manuscript," the lawyer, Charles Cooper, said.

Democrats want Bolton called as witness

The report drew Democratic demands that the Republican-led Senate, which is conducting a trial on whether to remove Trump from office after his December 18 impeachment by the Democratic-led House of Representatives, call Bolton as a witness.

Democrats have said they are eager to hear testimony by Bolton, who was involved, as his lawyer previously said, in "many relevant meetings and conversations" involving issues at the heart of Trump's impeachment.

Democrats need to win over at least four Senate Republicans to approve the calling of witnesses. Bolton said this month he was willing to testify in the trial if a Senate subpoena was issued.

Lawyers for Trump are scheduled on Monday to resume their defence in the impeachment trial stemming from his dealings with Ukraine. A showdown vote on calling witnesses could loom later in the week.

Trump denied the claims in a series of tweets early on Monday.

"I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens," Trump said in a tweet.

"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 said people could look at transcripts of his call, and statements by Ukraine President Zelenskyy that there was no pressure for such investigations to get the aid.

Peter Mathews, a political science professor at Cypress College, said Trump's attempt to frame the issue as a matter of national security and prevent Bolton's testimony was not a new strategy.

"So many presidents have used the national security camouflage to hide the truth," Mathews told Al Jazeera.

"If what Bolton is saying is true, it needs to be exposed so we can actually clean up our democracy and abide by the rule of law."