Donald Trump has asserted that his former chief strategist Steve Bannon "lost his mind" when he was fired form his White House role.
The US president's comments - made in a vitriolic statement against Mr Bannon - follows news that his former adviser has criticised him in a new book.
Mr Bannon, whose alt-right views have made him no stranger to controversy, reportedly called a meeting between Mr Trump's son and a Russian lawyer, which took place during the president's election campaign, "treasonous" and "unpatriotic".
That comment and several others delivered in acerbic style are relayed to the author Michael Wolff in his forthcoming book 'Fire and Fury: Inside the Trump White House', excerpts from which were published today by The Guardian newspaper.
Mr Trump's response has been to say of Mr Bannon: "When he was fired, he not only lost his job, he lost his mind."
Mr Bannon headed Mr Trump's election campaign in 2016 and went on to become a key adviser in his administration, until he was pushed out in August last year.
In his statement, Mr Trump also accuses Mr Bannon of leaking false information "to make himself seem far more important than he was" and claimed that he "pretends to have influence to fool a few people with no access an no clue, whom he helped write phony books".
After he fired Mr Banon in August, Mr Trump played down his former confidante's influence saying he had "very little to do" with the Trump campaign's success.
Mr Bannon has since returned to the right-wing website Breitbart News, where he is executive chairman.