Former National Security Adviser John Bolton, in his unpublished manuscript 'The Room Where It Happened' - as revealed earlier by The New York Times - claims that US President Donald Trump told him about his intention to freeze $391 million of approved military aid to Kiev until Ukrainian officials opened an investigation into the Bidens.

Trump, who lashed out at his former national security adviser, slamming the book as "nasty" and "untrue", on Wednesday tweeted "GAME OVER" and attached a piece from a Bolton interview dated August 2019, while the latter was still in Trump's employ, in which he did not mention a quid-pro-quo with Ukrainian President Volodymyr Zelensky - the primary accusation against the impeached US president.

In the interview clip released prior to Bolton's firing by Trump, the former characterized Trump's communications with Ukraine's leader as "cordial", and did not make mention of any form of misconduct. The interview contrasts with assertions Bolton makes in his forthcoming book.

The contradiction emerging after comparing drafts from Bolton's memoir and his earlier statements regarding Trump foreign policy has caused some in the US Senate to question Bolton's credibility as a possible witness, according to Fox News.

Bolton said earlier this month that he was ready to testify during the impeachment trial, if the US Senate issued a special subpoena for him.

Bolton did not testify before the Democrat-led House of Representatives, which in December 2019, after months of deliberations, impeached Trump on charges of abuse of power and obstruction of Congress over his dealings with Ukraine. 

Trump denies wrongdoing and has repeatedly dismissed the impeachment as a witch hunt aimed at reversing the outcome of the 2016 presidential election.

Trump fired Bolton in September 2019 over reported disagreements on US foreign policy issues related to Iran, North Korea, Venezuela and Cuba, among others.

"The Room Where It Happened" is reportedly scheduled for a March release date. Suspicions have been raised about where the leak to the New York Times originated from. Bolton, his publisher, and his agents denied any "coordination" with The New York Times.