US President Donald Trump appears to have confirmed he is under investigation for obstruction of justice in relation to the Russian meddling probe, claiming he is being investigated for firing FBI director James Comey by the man who told him to do it.

Key points: Donald Trump tweeted he was "being investigated for firing the FBI Director"

Donald Trump tweeted he was "being investigated for firing the FBI Director" But source claims the President was reacting to a report that Robert Mueller was looking into whether he obstructed justice

But source claims the President was reacting to a report that Robert Mueller was looking into whether he obstructed justice Mr Trump says nobody has been able to show "proof" of collusion with Russians

It was not clear whether the President was basing his tweet on direct knowledge that he is under investigation, or on reports this week that special counsel Robert Mueller is examining whether the President obstructed justice by firing Mr Comey last month amid the ongoing Russia investigation.

Mr Mueller was appointed special counsel to investigate Russian involvement in the 2016 presidential election and possible collusion by Trump campaign associates with the Russians, but there has been no indication that Mr Mueller told Mr Trump to fire Mr Comey.

The President tweeted on Friday morning that he was "being investigated for firing the FBI Director by the man who told me to fire the FBI Director".

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Mr Trump did not identify who "the man" was but appeared to be questioning the integrity of Deputy Attorney-General Rod Rosenstein — the Justice Department's number two official, who appointed Mr Mueller on May 17 and wrote a memo to Mr Trump that was critical of Mr Comey's performance after the former director was fired.

The Trump administration initially said Mr Rosenstein's letter was the reason the President fired Mr Comey on May 9, but Mr Trump later said he did so because of the "Russia thing".

Hours after Mr Trump's tweet, a source close to his outside legal team said the President did not intend his tweet to be confirmation of the investigation but rather was a reaction to a Washington Post story about the probe from Wednesday (local time).

The source spoke on condition of anonymity.

In an earlier tweet on Friday, Mr Trump wrote that nobody had been able to show any proof of "collusion with the Russians".

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Mr Comey told a Senate panel last week he believed Mr Trump fired him to undermine the FBI's Russia probe and testified that the President directed him in February to drop an investigation into former national security adviser Michael Flynn in relation to the Russia matter.

He also said it would be up to Mr Mueller to decide whether Mr Trump's action amounted to obstruction of justice, an act that could be cited in any effort in the Republican-led Congress to impeach him and remove him from office.

The White House had confirmed that Mr Mueller was among the candidates Mr Trump interviewed to replace Mr Comey.

A day later, Mr Rosenstein appointed Mr Mueller — who was at the helm of the FBI during the September 11 attacks — as special counsel.

'Single greatest witch hunt'

Friday's tweets are the latest in a week of angry social media responses by the President over a report by The Washington Post that Mr Mueller was looking into whether Mr Trump obstructed justice.

In a series of statements on Twitter on Thursday, Mr Trump repeated his assertion that Mr Mueller's investigation into Russian interference in the 2016 campaign was a "witch hunt" based on the "phony" premise of possible collusion between Russia and a cadre of Trump campaign associates.

The President complained the probe was unfair and wondered why his defeated Democratic opponent was not getting the same scrutiny.

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

"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," Mr 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 Twitter attacks came as Vice-President Mike Pence hired a personal lawyer to represent him in the intensifying investigation.

Mr Pence's office confirmed he had retained Richard Cullen, a former Virginia attorney-general and US attorney for the eastern district of Virginia, to assist "in responding to inquiries" from Mr Mueller.

Reuters/AP