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Donald Trump's former chief strategist Steve Bannon underwent hours of questioning by a congressional committee Tuesday probing alleged links between the president's election campaign and Russia.

It emerged that the former Trump aide has also been subpoenaed by the special prosecutor investigating allegations of meetings between the president's team and Russian officials.

Bannon was quizzed behind closed doors by the House of Representatives Intelligence Committee, the first time he has testified in the probe investigating whether Trump's campaign colluded with Russia in its bid to influence the 2016 US elections.

Subpoenaed

It came as Bannon was subpoenaed by Robert Mueller, the justice department special counsel, investigating the alleged collusion, The New York Times reported Tuesday.

That made Bannon the first person from Trump's inner circle to receive a grand jury subpoena from Mueller in the probe, which is also looking at whether the president has tried to obstruct the investigation.

When appearing before the Intelligence Committee, Bannon refused to answer a number of questions, citing "executive privilege" allowing the president to keep information from the public.

"Steve Bannon and his attorney asserted a remarkably broad definition of executive privilege," Representative Jim Himes, a Democratic member of the committee, said on CNN.

There were an awful lot of questions we weren't able to answer based on this novel theory of executive privilege. - Representative Jim Himes

"Now remember, it's the president who has the executive privilege and so they went back, conferred with the White House, and the White House said that anything that happened, any communications that happened while Steve Bannon was in the White House or during the transition, any communications were off limits," Himes said.

— Natasha Bertrand (@NatashaBertrand) " style="color:#fff;" class="twitter-post-link" target="_blank">Twitter Post . says WH had Bannon extend executive privilege to the period after he left the WH, to include his conversations with Trump during that period. Not a formal invocation of executive privilege by the White House. — Natasha Bertrand (@NatashaBertrand)

"There were an awful lot of questions we weren't able to answer based on this novel theory of executive privilege," he said.

"I'm glad to say that members of both parties pushed back hard against this unprecedented claim - what looks a lot like a gag order," said Himes.

The unrestricted testimony of Trump's estranged political strategist could be explosive.

Fire and Fury

As chief executive of the 2016 election campaign in its final months and later as a top policy advisor in the first seven months of the administration, Bannon had unprecedented access to the president.

A controversial book released last week, Fire and Fury: Inside the Trump White House, by Michael Wolff, quoted Bannon as saying that a pre-election meeting involving Trump's eldest son Donald Jr. and a Kremlin-linked Russian lawyer was "treasonous".

Wolff, who painted a picture of an erratic and poorly informed president, was given substantial access to the White House during Trump's first year by Bannon.

A hard-line nationalist who sought to shake up US domestic and foreign policy, Bannon, 64, was forced out as Trump's chief strategist in August.

The book has left him increasingly isolated in conservative circles.

Last week he stepped down from Breitbart News, which he had helped make a powerful conservative force.

Trump said Bannon had "lost his mind" and branded him "Sloppy Steve" via Twitter.