Deputy US Attorney General Rod Rosenstein, who appointed Special Counsel Robert Mueller and remains his most visible Justice Department protector, is expected to leave his position soon after William Barr is confirmed as attorney general, an individual familiar with the plans said on Wednesday.

Rosenstein has had oversight of the US Special Counsel's probe into alleged Russian interference in the 2016 presidential election and possible connections to Trump's campaign. Then-Attorney General Jeff Sessions, an early Trump supporter during the presidential campaign, had recused himself.

William Barr, Trump's pick to replace Sessions - who was fired soon after the November midterm congressional elections - is set to appear for a confirmation hearing next week before the Senate Judiciary Committee, which must weigh his nomination before the full Senate considers his approval.

If confirmed, Barr, who was US Attorney General under the late President George HW Bush from 1991 to 1993, would oversee the investigation led by Mueller, who was chosen by Rosenstein.

Critical steps in Mueller probe

The transition to Barr would come as critical steps in the Mueller investigation are expected to unfold in the weeks and months ahead, when the special counsel's office is expected to report its findings to the Department of Justice.

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Barr's nomination is likely to meet heavy scrutiny in light of the ongoing investigation, particularly from Democrats, following reports he had written a memo in June questioning the probe. Rosenstein has said the memo had no impact on the department's work.

On Wednesday, Republican Senator Lindsey Graham said Barr would permit Mueller to complete the investigation into Russia's role in the 2016 presidential election.

"I can assure you he has a very high opinion of Mr Mueller and he is committed to letting Mr. Mueller finish his job," Graham said.

Rosenstein will stay on to ensure a smooth transition with Barr, an official told Reuters news agency, adding that he saw his job as deputy as a two-year stint and is not being forced out.

Asked about Rosenstein's departure, first reported by ABC News, White House spokeswoman Sarah Sanders said she had not spoken to Rosenstein and would leave any announced departures to him or the president.

"Certainly, I don't think there's any willingness by the president or the White House to push him out," Sanders told Fox News in an interview.

Rosenstein has stayed on under Acting Attorney General Matthew Whitaker, whose controversial appointment sparked numerous legal challenges and raised questions about what role he would play regarding the investigation.

Rosenstein has been frequently criticised by Trump, who calls the Russia investigation a "witch-hunt" and denies any collusion with Moscow. Russia has also denied any interference in the election.

US intelligence agencies have found that Russia sought to sway the 2016 presidential vote to Trump over Democratic challenger Hillary Clinton.