Former FBI director Robert Mueller will lead the inquiry into alleged Russian interference in last year's election
Donald Trump has claimed he is facing "the single greatest witch hunt of a politician in American history".
President Trump tweeted the claim after yesterday's announcement that former FBI director Robert Mueller has been appointed as special counsel to investigate alleged Russian interference in last year's election campaign.
Deputy Attorney General Rod Rosenstein, who made the decision to appoint Mr Mueller, said: “My decision is not a finding that crimes have been committed or that any prosecution is warranted. I have made no such determination.
"What I have determined is that based upon the unique circumstances, the public interest requires me to place this investigation under the authority of a person who exercises a degree of independence from the normal chain of command."
In a statement released by the White House last night, President Trump said: “As I have stated many times, 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.
"In the meantime, I will never stop fighting for the people and the issues that matter most to the future of our country," he added.
However, in a pair of tweets today, he adopted a more hostile tone:
With all of the illegal acts that took place in the Clinton campaign & Obama Administration, there was never a special councel appointed!— Donald J. Trump (@realDonaldTrump) May 18, 2017
This is the single greatest witch hunt of a politician in American history!— Donald J. Trump (@realDonaldTrump) May 18, 2017
Mr Mueller's appointment was welcomed by both Republicans and Democrats.
House minority leader Nancy Pelosi said: “Former Director Mueller is a respected public servant of the highest integrity. The Trump Administration must make clear that Director Mueller will have the resources and independence he needs to execute this critical investigation.
"A special prosecutor is the first step, but it cannot be the last," she added.
The latest development came during a week in which the White House faced a series of fresh controversies.
On Monday, it was claimed that President Trump had disclosed 'highly classified information' to Russian officials during a meeting in the Oval Office.
Barely 24 hours later, The New York Times reported that President Trump urged the then FBI director James Comey to end an investigation into former national security adviser Michael Flynn.
Mr Comey was fired by President Trump last week, with the US president having since admitted the 'Russia thing' was on his mind when he made the decision.
A small number of Republicans and Democrats have raised the prospect of impeaching the US president following this week's revelations.
Trump claimed yesterday that "no politician in history" has been "treated worse or more unfairly" than he has.
Mr Comey, meanwhile, has been asked to testify before a Senate committee next week, while congressional officials have requested all records of meetings between the former FBI chief and President Trump.