John Bolton
John BoltonNoam Revkin Fenton/Flash90

Former US national security adviser John Bolton claims in his as yet unpublished memoir that President Donald Trump sought to tie hundreds of millions of dollars in aid to Ukraine to his requests for the country's leaders to investigate former Vice President Joe Biden and his son Hunter Biden, The Hill reported.

Sources familiar with Bolton's book told The New York Times on Sunday that, in it, he writes that Trump personally told him that $391 million in aid to Ukraine should be frozen until Ukrainian officials announced the investigations, including one into the Democratic National Committee.

The book, which does not have a publication date as of yet, has been submitted to the White House for review.

White House officials did not immediately return a request for comment from The Hill on the report.

The report comes amid the ongoing Senate debate on whether to allow witnesses in Trump's impeachment trial beyond those who spoke to House investigators in past months.

Senate Minority Leader Charles Schumer (D-NY), who has pressed Senate Majority Leader Mitch McConnell (R-KY) to allow such witnesses to testify, reiterated his demand following Sunday’s

"John Bolton has the evidence," he tweeted. "It’s up to four Senate Republicans to ensure that John Bolton, Mick Mulvaney, and the others with direct knowledge of President Trump’s actions testify in the Senate trial."

Bolton said earlier this month he is willing to testify, if he is subpoenaed, in the Senate's impeachment trial.

Bolton issued a statement after the courts did not rule whether he would be compelled to testify during the House's impeachment proceedings, saying he was trying to meet his "obligations both as a citizen and as former national security adviser."

The House sought his testimony but ultimately never subpoenaed Bolton, and Democrats withdrew their subpoena for his former deputy after it was challenged in court, as Democrats wanted to move forward with their impeachment probe and not wait for the court's decision.

Reports later said Bolton would be willing to testify before Congress in the impeachment investigations but only if a court rules on a congressional subpoena.