DONALD Trump has claimed he is the victim of the “greatest witch hunt of a politician in American history” following the appointment of a special counsel to investigate allegations his campaign collaborated with Russia to sway last year’s US presidential election.

The US justice department has appointed former FBI director Robert Mueller to lead the investigation. He will have sweeping powers and the authority to prosecute any crimes he uncovers.

The department had previously resisted calls from Democrats for an outside prosecutor.

The President tweeted: “With all of the illegal acts that took place in the Clinton campaign & Obama Administration, there was never a special councel appointed!”

He did not provide examples or evidence of any alleged “illegal acts”. Trump added: “This is the single greatest witch hunt of a politician in American history!”

In a written statement, Trump then said: “A thorough investigation will confirm what we already know – there was no collusion between my campaign and any foreign entity. I look forward to this matter concluding quickly.”

Mueller’s broad mandate gives him not only oversight of the Russia probe, but also “any matters that arose or may arise directly from the investigation”, which could include Trump’s firing of FBI director James Comey.

Mueller, Comey’s predecessor, was confirmed as FBI director days before the September 11, 2001, attacks. He was so valued that former president Barack Obama asked him to stay on two years longer than his 10-year term.

Republicans have largely stood behind Trump in the first months of his presidency as the FBI and congressional investigations into Russia's election meddling intensified.

However, they have grown increasingly anxious since Trump sacked Comey, who had been leading the bureau's probe - especially after Comey's associates said he had notes from a meeting in which Trump asked him to shut down the investigation into the Russian ties of his former national security adviser, Michael Flynn.

US House Speaker Paul Ryan said Mueller's appointment was consistent with his goal of ensuring that "thorough and independent investigations are allowed to follow the facts wherever they may lead".

Republican Jason Chaffetz of Utah, chairman of the House Oversight Committee, said Mueller was a "great selection. Impeccable credentials. Should be widely accepted".