The Justice Department has appointed former FBI director Robert Mueller as a special counsel to oversee a federal investigation into potential co-ordination between Russia and the Trump campaign during the 2016 presidential election.

The appointment on Thursday (NZT) came amid a growing Democratic outcry for someone outside the Justice Department to handle the politically charged investigation.

It followed the revelation Tuesday that fired FBI director James Comey had written in a memo that Trump, in a February meeting in the Oval Office, had asked him to end an investigation into former national security adviser Michael Flynn. The White House has denied that account.

REUTERS US President Donald Trump says he's been treated more unfairly than any politician in history.

Mueller was appointed FBI director in 2001 and led the FBI through the aftermath of the September 11 terror attacks. He retired in 2013.

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The Justice Department said Mueller had resigned from his job at a private law firm to take the job of special counsel.

LARRY DOWNING/REUTERS Former FBI Director Robert Mueller will take the lead on the Trump-Russia probe.

The department declined to explain the decision-making involved in the appointment.

REACTION

House Republicans had mixed reactions to the surprise announcement.

KEVIN LAMARQUE/REUTERS Earlier Trump complained in a commencement address that "no politician in history" has been treated worse by his foes.

Oversight Committee Chairman Jason Chaffetz of Utah said Mueller was a "great selection.Impeccable credentials. Should be widely accepted."

But Republican Peter King of New York is expressing concern over the wide purview special prosecutors have.

King says, "I'm worried with all special counsels because there's no control over them and they can abuse their power."

RNZ Washington Post reporter Greg Miller who broke the story of Trump's disclosure of classified material says Trump officials have gone into hiding.

In the 1990s, Democrats insisted that independent counsel Kenneth Starr, who investigated former President Bill Clinton, overstepped his authority.

- AP AND AAP