US President Donald Trump has lashed out on Twitter  over reports he is under investigation for possible obstruction of justice, saying he is the victim of a “witch hunt”.

The Washington Post, citing unnamed officials, reported on Thursday that special counsel Robert Mueller is investigating the President for possible obstruction of justice.

If true, the investigation represents an expansion of Mr Mueller’s original probe into links between Moscow and the 2016 Trump presidential election campaign.

You are witnessing the single greatest WITCH HUNT in American political history – led by some very bad and conflicted people! #MAGA

— Donald J. Trump (@realDonaldTrump) June 15, 2017

“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 tweeted, later repeating his accusation that the probe is a “witch hunt”.

The President than went on to renew his attack on his presidential election opponent Hillary Clinton and her husband, former US president Bill Clinton.

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

— Donald J. Trump (@realDonaldTrump) June 15, 2017

Crooked H destroyed phones w/ hammer, 'bleached' emails, & had husband meet w/AG days before she was cleared- & they talk about obstruction?

— Donald J. Trump (@realDonaldTrump) June 15, 2017

A source familiar with the Mueller investigation, who confirmed the Washington Post report, told Reuters an examination of possible obstruction of justice charges was “unavoidable” given testimony by former FBI director James Comey.

The source said the issue may not become the main focus of the probe.

Mr Comey, who was fired on May 9, told the Senate Intelligence Committee last week he believed President Trump dismissed him to undermine the FBI’s Russia probe.

He said he believed Mr Trump had directed him to drop a related agency investigation into the president’s former national security adviser, Michael Flynn.

The obstruction of justice investigation into Mr Trump began days after Mr Comey was fired, according to people familiar with the matter, the Washington Post said.

Mr Trump was strongly critical of some of Mr Comey’s testimony but said Mr Comey had vindicated him by saying while he was at the agency, Mr Trump was not the subject of the FBI’s Russia probe.

Special counsel Mueller is following two major lines of investigation, a  source told Reuters.

The first is into whether anyone on Mr Trump’s presidential campaign or associated with it, with him or with any of his businesses, may have had any illegal dealings with Russian officials or others with ties to the Kremlin.

The second, the source continued, is whether any potential offences were committed, Mr Trump or others attempted to cover them up or obstruct the investigation into them.

Examining such possible charges will allow investigators to interview key administration figures including Attorney General Jeff Sessions, Deputy Attorney General Rosenstein and possibly President Trump himself, said the source familiar with the Mueller investigation.

While a sitting president is unlikely to face criminal prosecution, obstruction of justice could form the basis for impeachment.

The White House has denied any collusion, and President Trump has repeatedly complained about the probe, saying Democrats cannot accept his election win.

On Thursday, Russian President Vladimir Putin said Mr Comey had presented no evidence to prove that Moscow meddled in the US election, adding that Washington had tried to influence Russian elections “year after year”.

Mr Putin echoed Mr Trump’s criticism of Mr Comey, saying it was “very strange” a former FBI chief leaked details of his conversations with the US president to the media through a friend.

– With AAP