The White House is pushing back against the publication of a book by former national security adviser John Bolton, citing security concerns.
The National Security Council said the book had “top secret” details that must be removed, a claim Mr Bolton rejects.
A manuscript of the book reportedly claims that President Donald Trump linked freezing military aid to Ukraine in exchange for a political favour.
The book row comes as the questioning period of the impeachment trial begins.
On Friday, senators will hold a key vote on whether or not to allow witnesses in the trial. Democrats are demanding Mr Bolton testify in the wake of these claims, and some Republicans have also suggested they would like to hear from the former adviser.
The president has denied all the allegations against him.
National Security Council (NSC) senior director Ellen Knight said in a letter to Mr Bolton’s attorney that following a preliminary review, the “manuscript appears to contain significant amounts of classified information”.
“It also appears that some of this classified information is at the top-secret level,” Ms Knight wrote. She went on to warn that the “manuscript may not be published or otherwise disclosed without the deletion of this classified information”.
A senior White House official said on Wednesday that the letter was not a “threat” to the former national security adviser. But President Trump’s defence in his impeachment trial has been imperilled by the reported revelations in the book.
The manuscript is said to contain an account of the president directly telling Mr Bolton that he would withhold US military aid to Ukraine to pressure Ukraine to help him win the 2020 US election.
Senior figures in Mr Trump’s Republican Party are now fighting to prevent Mr Bolton being called as a witness at the impeachment trial, while a small handful say they will vote for witnesses to appear.
A copy of Mr Bolton’s manuscript was reportedly sent to the White House weeks ago for review. A White House official said the letter was sent to Mr Bolton prior to revelations from his book being reported by the New York Times last week.
The president lashed out at his former national security adviser on Twitter on Wednesday, calling his book “nasty & untrue”. He insisted he “NEVER” told John Bolton the military aid was tied to getting dirt from Ukraine on his domestic political rivals.