US President Donald Trump has said that he is under investigation for obstruction of justice, confirming reports that Special Counsel Robert Mueller is overseeing a probe into the alleged Russian interference in the American presidential election.

I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt, the president wrote in his tweet on Friday.

Trump did not make it clear whom his tweet was referring to. However, it is likely that he was tweeting about Deputy Attorney General Rod Rosenstein, or Mueller who is examining whether the president obstructed justice by firing ex-FBI Director James Comey last month amid the ongoing Russia investigation.

Last month, Rosenstein wrote a memo to the president raising concerns over Comey's performance. However, Trump later asserted that he had already made the decision himself to fire Comey.

It was confirmed by the White House that Mueller was among the candidates who Trump interviewed to replace Comey. A day later, Rosenstein appointed Mueller as special counsel.

In a tweet earlier Friday, Trump said nobody has been able to show any proof about his collusion with Moscow after months of investigations.

Trump wrote, After 7 months of investigations & committee hearings about my 'collusion with the Russians,' nobody has been able to show any proof. Sad!

This is a political fight and he's going to fight it, a White House official said regarding president's Friday's tweets.

In a series of tweets on Thursday, the president repeated his assertion that Mueller's investigation into Russian interference in the 2016 campaign is a WITCH HUNT based on the phony premise of possible collusion between Russia and a team of Trump campaign associates.

They made up a phony collusion with the Russians story, found zero proof, so now they go for obstruction of justice on the phony story. Nice, Trump wrote in his first tweet. You are witnessing the single greatest WITCH HUNT in American political history - led by some very bad and conflicted people! #MAGA.

The president made a tweet about his defeated Democratic opponent complaining that the probe is unfair.

Why is that Hillary Clintons family and Dems dealings with Russia are not looked at, but my non-dealings are? he asked.

The shift toward probing Trump began days after Trump forced former FBI chief James Comey to quit last month. Comey was fired by the president on May 9 while he was overseeing the investigation into potential Russian interference into last year's election and potential collusion with the Trump presidential campaign.

Trump has vehemently denied any collusion between himself or any of his associates and Russia.