Former White House aide Steve Bannon "lost his mind" after he lost his job at the White House, US President Donald Trump has said.
The US president disavowed Mr Bannon after he was quoted in a new book describing a meeting between Mr Trump's son and a group of Russians as "treasonous".
The Russians had offered Donald Trump Jr damaging information on Hillary Clinton at the June 2016 meeting.
A federal counter-intelligence investigation of Moscow's contacts with the Trump campaign team has cast a long shadow over the administration.
Mr Bannon's quote appears in a new book by journalist Michael Wolff.
White House spokeswoman Sarah Sanders said the Wolff book on the Trump administration was filled with "false and misleading accounts".
"Participating in a book that can only be described as a trashy tabloid fiction exposes their sad desperate attempts at relevancy".
President Trump's statement said Mr Bannon had "nothing to do with me".
"When he was fired, he not only lost his job, he lost his mind.
"Now that he is on his own, Steve is learning that winning isn't as easy as I make it look. Steve had very little to do with our historic victory, which was delivered by the forgotten men and women of this country... Steve pretends to be at war with the media, which he calls the opposition party, yet he spent his time at the White House leaking false information to the media to make himself seem far more important than he was.
"It is the only thing he does well. Steve was rarely in a one-on-one meeting with me and only pretends to have had influence to fool a few people with no access and no clue, whom he helped write phony books."
Mr Bannon was the architect of the 'America First' message in the election campaign and during the early days of the new administration he was one of many who were either fired or left the White House.
The president's comments came hours after the explosive new book, Fire and Fury: Inside the Trump White House, revealed that Mr Bannon called Mr Trump Jr's meeting "treasonous" and "unpatriotic".
Speaking to the author, Mr Wolff, Mr Bannon said of the investigation into whether there was any collusion between Russia and the Trump campaign:
"They're going to crack Don Junior like an egg on national TV."
- BBC / Reuters / RNZ