US President Donald Trump's former advisor Steve Bannon has been subpoenaed by Special Counsel Robert Mueller to testify before a grand jury into alleged ties between Russia and Trump's 2016 presidential campaign, the New York Times has reported
It was the first time Mr Mueller is known to have used a subpoena against a member of Trump's inner circle, the Times said.
A spokesman for Mr Mueller's office declined to comment. Bill Burck, a lawyer for Mr Bannon, could not immediately be reached for comment.
Mr Bannon, a champion of the "America First" agenda, was among Mr Trump's closest aides during the 2016 election campaign, the presidential transition and during his first months in office.
But the pair had a bitter public falling out over comments Mr Bannon made to author Michael Wolff for his recent book "Fire and Fury: Inside the Trump White House."
In the book, Mr Bannon is quoted as describing a June 2016 meeting between Mr Trump associates, including the president's son Donald Trump Jr, his son-in-law, Jared Kushner, and a Russian lawyer, as "treasonous" and "unpatriotic."
The meeting came after Donald Trump Jr was told in an email that the Russian government had compromising information about Democratic presidential candidate Hillary Clinton, to which he replied: "I love it."
Mr Bannon was fired by the White House in August and returned to the right-wing news website Breitbart News. He continued to speak with President Trump and tried to promote the president's agenda.
But Mr Trump accused Mr Bannon of having "lost his mind" when news of his comments to Michael Wolff surfaced earlier this month. Six days later, Mr Bannon stepped down from his post as executive chairman at Breitbart News.
Mr Mueller's subpoena, which was issued last week, could be a pressure tactic to induce Mr Bannon to cooperate fully with his investigation, the Times reported.
Separately, Mr Bannon was meeting with the House of Representatives Intelligence Committee. He is the latest high-profile figure to testify before the panel as part of its investigation into claims of Russian interference in the US election.
Russia has denied meddling in the election and President Trump has denied there was any collusion between his campaign and Moscow.
Later this week, the panel will hear from former Trump campaign manager Corey Lewandowski.
White House Communications Director Hope Hicks, who served as Mr Trump's spokeswoman during his presidential campaign, is also expected to be questioned by the committee this week.
Democrats on the committee have accused Republicans of rushing to wrap up the probe to help give the president political cover, despite their requests to interview more witness. Republicans have denied the charge.
Meanwhile a federal judge has delayed setting a trial date for Mr Trump's former campaign manager Paul Manafort, who faces charges stemming from Mr Mueller's probe.
Defense lawyers said the government had not produced all the evidence it gathered and they needed more time to study it and file motions.
"We have limited resources," Mr Manafort’s attorney Kevin Downing said. "We're not big law firms."
The government had sought a May start to the trial. However Judge Amy Berman Jackson said a trial for Mr Manafort and his business partner Richard Gates might not start until the autumn.
