"He lost his mind." Donald Trump unleashes on his former chief strategist.

President Donald Trump has issued a damning statement about former White House chief strategist Steve Bannon, who in a new book was quoted as calling a June 2016 meeting with a group of Russians attended by the president’s eldest son “treasonous”.

“Steve Bannon has nothing to do with me or my Presidency. When he was fired, he not only lost his job, he lost his mind,” the statement read.

“Steve was a staffer who worked for me after I had already won the nomination by defeating seventeen candidates, often described as the most talented field ever assembled in the Republican party.

"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. Yet Steve had everything to do with the loss of a Senate seat in Alabama held for more than thirty years by Republicans. Steve doesn't represent my base -- he's only in it for himself.

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"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.

"We have many great Republican members of Congress and candidates who are very supportive of the Make America Great Again agenda. Like me, they love the United States of America and are helping to finally take our country back and build it up, rather than simply seeking to burn it all down."

64-year-old Steven Bannon, a former US Navy officer, Goldman Sachs investment banker and Hollywood movie producer, was fired by Trump in August.

Bannon was a powerful and controversial figure known for far-right political views, and was a force behind some of Trump’s most contentious policies, including a travel ban on people from several Muslim-majority nations.