He met with special counsel Robert Mueller, who leads the inquiry, over two days this week.

Mr Bannon was chairman of Mr Trump's presidential campaign before becoming a top White House official.

But he left his post in August amid reports of tension with other aides.

Mr Mueller is leading an investigation into whether the Trump campaign colluded with Russia to influence the US election. Both Moscow and Mr Trump deny this.

Investigators believe Mr Bannon may hold crucial information on the Trump administration, including details on why former FBI Director James Comey was fired by the president.

Mr Comey has previously said he has "no doubt" that he was fired by Mr Trump to "change" the Russia investigation, a charge the White House denies.

It is not clear what Mr Bannon was questioned about, but he is reported to have spent around 20 hours speaking with Mr Mueller's team.

A source close to the process though told the Associated Press that Mr Bannon answered every question put to him.

This inquiry is running alongside four similar ones being conducted by politicians in Congress, and Mr Bannon spoke to one of them on Thursday.

He appeared in front of the House intelligence committee as part of their own, separate, investigation into possible ties between the Trump campaign and Moscow.

But he frustrated the committee when he refused to answer their questions, opting instead to answer 25 pre-written questions that had been approved by the White House.

It led top Democrats to call for contempt proceedings against Mr Bannon.

"There was a refusal to answer any questions that would have brought out the full facts. That is not how privilege works. That's how stonewalling works," the panel's Democratic leader Adam Schiff said.

In a tweet, Democratic member Joaquin Castro also accused Mr Bannon of stonewalling.

Steve Bannon stonewalled the intel committee today. Sure seems like he’s covering up something(s). He should be held in contempt. #Bannon

Republican Representative Mike Conaway said members of the panel were unhappy with Mr Bannon's responses.

"He did not answer all the questions we'd like answered, so there was frustration among committee members with respect to that," he said.

It is not the first time Mr Bannon has been criticised by the panel.

He voluntarily met with the same committee in January and refused to answer their questions over a 10-hour period.

The committee then issued a subpoena compelling him to return for a second time.

But Mr Bannon was reportedly advised by President Trump to invoke executive privilege, a prerogative allowing him to withhold information.

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