U.S. President Donald Trump threatened former chief strategist Steve Bannon with legal action over "defamatory" statements about Trump's son and son-in-law over a meeting with Russians during the 2016 presidential campaign.

Trump cut ties with Bannon on Wednesday, saying his former adviser had "lost his mind" in a blistering statement issued after reports of disparaging comments by Bannon in a forthcoming book, "Fire and Fury: Inside the Trump White House" by Michael Wolff.

In the book, Bannon described a June 2016 meeting with a group of Russians at Trump Tower in New York as "treasonous" and "unpatriotic." The meeting, held after the Russians promised damaging information on Democratic presidential candidate Hillary Clinton, was attended by Donald Trump Jr., Trump son-in-law Jared Kushner and Paul Manafort, who was the campaign manager at the time.