Steve Bannon, former chief strategist to US President Donald Trump, has been summoned to testify before a grand jury, US media report.

Steve Bannon, former chief strategist to President Trump, has stepped down from Breitbart, the far-right website he returned to over the summer as executive chairman after being ousted from the White House.

Donald Trump's former strategist Steve Bannon backed away Sunday from derogatory comments ascribed to him about Trump's son in a new book that sparked White House outrage and could threaten Bannon's influence as a would-be conservative power broker.

Michael Wolff's decision to publish 'Fire and Fury' after Trump's lawyers filed a cease-and-desist letter was an unprecedented move. Here's why. USA TODAY

DONALD Trump has slammed his former right-hand man Steve Bannon after the former aide called a meeting between the president’s son and the Russians “treasonous”.

President explodes as book quotes his ex-adviser Bannon saying a Trump Jr. meeting was 'treasonous'

* Former White House strategist quoted in Fire and Fury, by Michael Wolff
* Bannon: ‘They’re going to crack Don Junior like an egg on national TV

Bannon clashed with what he has described as a "globalist" White House faction.

"These guys are a collection of clowns"

WHITE House chief strategist Steve Bannon knows why spokesman Sean Spicer is holding off-camera press briefings — “Sean got fatter.”

Trump is said to have implored his staff to resolve their differences.

President-elect Donald Trump’s choice of Steve Bannon