US President Donald Trump's fellow Republicans in the US Senate have come under fresh pressure to allow witnesses and new documents in his impeachment trial after a news report that a former top aide, John Bolton, has written a book manuscript that undercuts Mr Trump's versions of events in the Ukraine affair.

Republican Senator Mitt Romney, a sometime critic of Mr Trump, said there was a growing likelihood that at least four Republican senators would vote to call for Mr Bolton to testify in the trial, which would give Democrats the votes necessary to summon the former national security adviser.

Senate Republicans thus far have refused to allow any witnesses or new evidence in the trial that will determine whether Mr Trump is removed from office.

The president's legal team resumes its defense of Mr Trump this evening.

The New York Times cited the manuscript of an unpublished book by Mr Bolton as saying that Mr Trump told him he wanted to freeze security aid to Ukraine until the eastern European country's government helped with politically beneficial investigations into Democrats, including former Vice President Joe Biden and his son, Hunter Biden.

If confirmed, the report would add weight to Democrats' accusations that Mr Trump used the $391 million in aid - approved by the US Congress to help Ukraine combat Russia-backed separatists - as leverage to get a foreign country to help him dig up dirt on a domestic political rival.

Mr Biden is a leading contender for the Democratic nomination to face Mr Trump in the presidential election on 3 November this year.

Mr Trump has denied telling Mr Bolton that he sought to use the aid to pressure Ukrainian President Volodomyr Zelenskiy to investigate the Bidens on unsubstantiated corruption allegations.

Hunter Biden worked for a Ukrainian energy firm while his father was US vice president.

Mr Bolton left his post in September, Mr Trump saying he fired him while Mr Bolton said he quit.

"I think it's increasingly likely that other Republicans will join those of us who think we should hear from John Bolton," Mr Romney told reporters.

Another moderate Republican senator, Susan Collins, said the reports regarding Mr Bolton's book "strengthen the case for witnesses."

Senate Judiciary Committee Chairman Lindsey Graham, a Republican ally of Mr Trump, said he would support issuing a subpoena to obtain Mr Bolton's manuscript to see if it should be added to the record, a CNN reporter said on Twitter.

The Democratic-led House of Representatives impeached Mr Trump last month on charges of abuse of power in his dealings with Ukraine and obstruction of Congress, setting up the trial in the Republican-led Senate.

Mr Trump is expected to be acquitted in the 100-seat Senate, where Republicans hold 53 seats and a two-thirds vote is required to convict and remove a president from office. No Republican senator has voiced support for his removal.

Mr Trump denied telling Mr Bolton he was seeking something in return for unfreezing the Ukrainian aid, which eventually was provided in September after the controversy became public.

"I haven't seen the manuscript, but I can tell you nothing was ever said to John Bolton," Mr Trump told reporters outside the White House ahead of a meeting with Israeli Prime Minister Benjamin Netanyahu.

Democrats have demanded that the Senate call Mr Bolton as a witness.

"It completely blasts another hole in the president's defense," said Representative Adam Schiff, the head of the House Democratic team of "managers" who are presenting the prosecution case against Mr Trump.

"For every senator, Democrat and Republican, I don't know how you can explain that you wanted a search for the truth in this trial and say you don't want to hear from a witness who had a direction conversation about the central allegation in the articles of impeachment," Mr Schiff told CNN.

The issue of whether to call new witnesses - including Mr Bolton - might be resolved in a Senate vote on Friday or Saturday.