The White House has sent a letter to John Bolton’s lawyer objecting to “classified information” at the Top Secret level in the manuscript of his forthcoming book.
The finding could delay the book’s publication if Mr Bolton is forced to revise the draft.
But the former US national security adviser and his lawyer insist that the book does not contain any classified information.
The National Security Council’s (NSC) senior director for records, access and information security management warned Mr Bolton’s lawyer that the manuscript contains “significant amounts of classified information” and may not be published without deletion of that information.
The book is under pre-publication review by the NSC, a process that senior White House officials agree to in order to gain access to classified information.
Mr Bolton left the White House last September and says he resigned. US President Donald Trump says he was fired.
Mr Trump’s impeachment trial is shifting to questions from senators, as Republicans lack the votes to block witnesses and face a potential setback in their hope of ending the trial with a quick acquittal.
Despite Mr Trump’s defence team’s plea for it to “end now” Senate majority leader Mitch McConnell privately told senators he does not yet have the votes to brush back Democratic demands for witnesses amid Mr Bolton’s revelations.
Republican senators are trying to figure out a way to deal with fallout from Mr Bolton’s book, which provides a potential eyewitness account of Mr Trump’s actions at the heart of the impeachment charges.
But they are warned by Republican party leaders that calling Mr Bolton as a witness could entangle the trial in lengthy legal battles and delay Mr Trump’s expected acquittal.
One key Republican, senator Susan Collins, gave fresh momentum to a one-for-one witness deal saying it’s “very important that there be fairness, that each side be able to select a witness or two”.
But Democrats dismissed those offers, especially as Republicans want to draw Democratic presidential candidate Joe Biden’s son, Hunter, deeper into the proceedings.
“It’s irrelevant. It’s a distraction,” said Senate Democratic leader Chuck Schumer.
Mr Bolton writes in a forthcoming book that Mr Trump told him he wanted to withhold military aid from Ukraine until it helped with investigations into Mr Biden.
That assertion, if true, would undercut a key defence argument and go to the heart of one of the two articles of impeachment against the president.
“I think Bolton probably has something to offer us,” said Republican senator Lisa Murkowski.
Mr Trump disagreed in a tweet in which he complained that Mr Bolton, after he left the White House, “goes out and IMMEDIATELY writes a nasty & untrue book. All Classified National Security”.
The uncertainty about witnesses arises days before crucial votes on the issue. In a Senate split 53-47 in favour of Republicans, at least four Republican senators must join all Democrats to reach the 51 votes required to call witnesses, decide who to call or do nearly anything else in the trial. Several Republicans are apparently ready to join Democrats in calling witnesses.
One Democrat, the centrist senator Joe Manchin, said he’d would not have a problem hearing from Hunter Biden, who was on the board of a Ukrainian gas company, but doubted it will happen.
The witness swaps seem likely to fail as most Republican senators do not want to call Mr Bolton and most Democrats would rather avoid dragging the Bidens further into the impeachment proceedings. The Bidens were a focus of defence arguments though no evidence of wrongdoing has emerged.