The former-Attorney General Eric Holder has questioned whether Bill Barr's investigation into alleged misconduct by US agencies at the start of the Russia probe can possibly be impartial when it was ordered by Donald Trump.

Holder, 68, who served during Barack Obama's presidency, said the current attorney general risked 'paying a price' and sacrificing his credibility on Tuesday.

Barr assigned U.S. Attorney John Durham to pursue a probe into whether there was misconduct by the intelligence community at the beginning of the Russia investigation.

Speaking at an event in Fairfax, Virginia, Holder explained that Barr needed to be 'a little more sensitive to the appearance' given by having Trump involved.

He told Fox News: 'To see how the president is now involved to help the attorney general in that effort gives me pause.'

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Eric Holder, the former-Attorney General to Barack Obama, said that the probe into alleged interference could not be seen as impartial and would damage credibility

Current Attorney General Bill Barr (right) launched the probe into the misconduct during the 2016 election as ordered by President Donald Trump

Holder added: 'You have to not only be substantively neutral - you have to appear to be neutral when you are the attorney general of the United States. And I fear that he has crossed a political line'

The probe led by Durham will analyse whether there was improper government surveillance on the Trump campaign in 2016.

It will also seek to discover whether the Democrats colluded with foreign agents.

Holder added that probe was not considered 'neutral,' then it risked damaging Barr's reputation and that of the Department of Justice, including the FBI.

He went onto ask if the probe would discover anything 'the inspector general wasn't already doing?'

Holder said: 'It seems to be this is duplicative in a lot of ways. And I think kind of unnecessary.

'But, the attorney general has made that determination and I think is paying a price for it -- both in terms of questioning whether he's acting as the president's lawyer as opposed to the attorney general, and then it has a negative impact on the Justice Department as well.'

A DOJ spokesperson told Fox that Durham was 'gathering information from numerous sources, including a number of foreign countries.'

She added: 'At Attorney General Barr's request, the President has contacted other countries to ask them to introduce the Attorney General and Mr. Durham to appropriate officials.'

Holder acknowledged that it was not unusual for the president to introduce their attorney general with other officials.

But he added that it would typically be 'reserved for the most important investigations' like a 'direct impact on national security.'

Holder said: 'You don't involve heads of state in these matters unless the consequences are really significant.'

U.S. Attorney John Durham (left) has been assigned to investigate the allegations. In response to the comments, former-Senior Adviser to George W. Bush, Karl Rove (right), said Holder should not comment about the role of an AG as he was not 'impartial'

Holder's comments have triggered backlash from Karl Rove, the former senior adviser to George W. Bush.

Rove told Fox: 'Every attorney general I can remember, when they left the office except for Robert Kennedy, sort of melded back into the legal community - what is this guy doing?'

He accused Holder of 'leading a nationwide effort to attack Republicans' and added that he has no right to 'lecture' anyone about impartiality.

Rove alleged that during the Obama administration Holder ignored complaints of voter intimidation at the polls for the 2008 election by the Black Panther Party.

He said: 'This guy has no right whatsoever to lecture us about impartiality particularly on the part of an attorney general.'