A Democratic push to force Republicans to accept witnesses at Donald Trump's impeachment trial in the US Senate appeared to be flagging on Wednesday, raising the possibility the president could be acquitted as early as Friday.

As senators spent the day posing questions to both the Trump legal team and the Democratic managers of the trial, the White House objected to the planned publication of a book by former national security adviser John Bolton, who is said to have depicted Trump as playing a central role in pressuring Ukraine to investigate Democratic rival Joe Biden.

US Senator John Barrasso, the No 3 Republican in seniority, said it was possible the trial could end on Friday without Democrats achieving their goal of having witnesses called to testify.

"The momentum is clearly in the direction of moving to final judgment on Friday. That vote will be Friday. We still have a couple of members who said they want to listen to the answers to questions, but that's where the momentum is," Barrasso said.

Asked when on Friday the vote might take place to settle the debate over witnesses and move to either acquit or convict Trump, Barrasso said probably Friday afternoon or late that day.

Other Republican senators were predicting a similar outcome in conversations with reporters during breaks in the trial on Wednesday. They said some of their colleagues still wanted to hear answers to their questions through Thursday, however.

Democrats have sought to persuade at least four Republican senators to vote with them in favour of witnesses to assure a majority vote in the Republican-led 100-seat Senate. The Democratic-led House of Representatives approved the two articles of impeachment in December.

While the Senate is expected to acquit Trump and leave him in office no matter what happens, allowing witnesses such as Bolton could inflict political damage on the president as he seeks re-election on Nov 3.

Bolton, who left the White House in September after several sharp disagreements with Trump over the direction of foreign policy, was again a central figure on Wednesday.

The New York Times has reported that Bolton wrote in his book that the president told him he wanted to freeze US$391 million in security aid to Ukraine until Kiev pursued investigations into Democrats, including Biden and the former vice-president's son Hunter.

'Classified information'

A letter from the White House National Security Council to Bolton's attorney said that based on a preliminary review, the manuscript appeared to contain "significant amounts of classified information".

Some material was deemed top secret and could "cause exceptionally grave harm to the national security", according to the letter, which was dated Jan 23.

"Under federal law and the nondisclosure agreements your client signed as a condition for gaining access to classified information, the manuscript may not be published or otherwise disclosed without the deletion of this classified information," the letter said.

The manuscript was submitted to the White House for pre-publication review, a process that civil libertarians have said gives the government too much power to censor speech.

Democrats believe Bolton (above), a Republican foreign policy hawk, could help them solidify their case against the president and want to call him and a small number of other officials to testify. Trump's fellow Republicans in the Senate have resisted the idea of having any witnesses.

Bolton's allegations go to the heart of the impeachment charges. Democrats have said Trump abused his power by using the security aid - passed by Congress to help Ukraine battle Russia-backed separatists - as leverage to get a foreign power to smear a political rival. Trump's legal team has argued that the evidence supporting that allegation is based on hearsay.

A Bolton spokesperson had no comment on the White House letter. Neither his publisher nor lawyer immediately responded to requests for comment.

A vote on witnesses

Removing Trump from office would require a two-thirds majority. There are 53 Republican senators and none has publicly advocated removal.

Trump lashed out at Bolton on Twitter on Wednesday, saying Bolton "couldn't get approved for the Ambassador to the UN years ago, couldn't get approved for anything since, 'begged' me for a non-Senate approved job" and added that "if I listened to him, we would be in World War Six by now."

Trump added that Bolton "goes out and IMMEDIATELY writes a nasty & untrue book. All Classified National Security. Who would do this?"

Trump has said he fired Bolton. Bolton, who served as a temporary "recess appointee" as American ambassador to the United Nations under Republican former President George W Bush, has said he quit.

Responding to a question by top Senate Democrat Chuck Schumer, lead House prosecutor Adam Schiff said there would be no way to have a fair trial without witnesses.

"And when you have a witness as plainly relevant as John Bolton who goes to the heart of the most serious and most egregious of the president's misconduct, who has volunteered to come and testify - to turn him away, to look the other way, I think is deeply at odds with being an impartial juror," Schiff said.

- Reuters