Thu May 18, 2017 3:37PM
US President Donald Trump speaks during the US Coast Guard Academy Commencement Ceremony in New London, Connecticut, May 17, 2017. (Photo by AFP)
US President Donald Trump speaks during the US Coast Guard Academy Commencement Ceremony in New London, Connecticut, May 17, 2017. (Photo by AFP)

US president Donald Trump has condemned an investigation into possible collusion between his 2016 election campaign and Russia as the “greatest witch hunt” in the US history.

"This is the single greatest witch hunt of a politician in American history," he tweeted on Thursday.

The tweet came after the US Justice Department appointed a special counsel to oversee the investigation into alleged links between the Trump team and Moscow.

“With all of the illegal acts that took place in the Clinton campaign & Obama Administration, there was never a special councel (sic) appointed!” Trump wrote.

Former FBI director Robert Mueller was appointed by Deputy Attorney General Rod Rosenstein to lead the probe which will also look into “related matters.”

“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," Rosenstein said in a statement on Wednesday.

Mueller was at the helm of the Federal Bureau of Investigation for 12 years, between 2001 and 2013.

Last week, the US media reported that Trump had asked ousted FBI Director James Comey to drop the investigation a day after former national security adviser Michael Flynn was forced out in mid-February over concerns about his contacts with Russian officials.

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In a statement released by the White House, 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.”

Trump dismissed Comey last week citing his handling of the FBI investigation into Hillary Clinton's use of a private email server while at the State Department.

The dismissal sent shockwaves through Washington, with critics accusing Trump of trying to thwart the FBI investigation into Russia's alleged interference in the US presidential election and his possible collusion with the Kremlin.