WASHINGTON (REUTERS) - US President Donald Trump lashed out on Thursday (June 15) after a report that he was under investigation into possible obstruction of justice, dismissing as "phony" the notion his campaign colluded with any Russian effort to sway the 2016 US presidential election.

"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 on Twitter, later repeating his accusation that the probe is a "witch hunt."

The Washington Post, citing unidentified officials, reported on Wednesday that special counsel Robert Mueller is investigating the Republican president for possible obstruction of justice.

Former Federal Bureau of Investigation Director James Comey told Congress last week he believed Trump fired him in May to undermine the agency's Russia investigation.

Mueller was named by Deputy Attorney-General Rod Rosenstein eight days after Comey's dismissal to lead the Russia investigation as a special counsel, a position created to conduct investigations when a normal Justice Department probe would present a conflict of interest or in other extraordinary circumstances.

A source familiar with the Mueller investigation confirmed the Post report, saying an examination of possible obstruction of justice charges was "unavoidable" given Comey's testimony, although the issue may not become the main focus of the probe.

Examining such possible charges will allow investigators to interview key administration figures including Attorney General Jeff Sessions, Rosenstein and possibly Trump himself, the source told Reuters.

While he was strongly critical of some of Comey's testimony to a Senate panel, the president said last week that the former FBI chief had vindicated him when he said that while he was at the agency, Trump was not the subject of the FBI's Russia probe.

While a sitting president is unlikely to face criminal prosecution, obstruction of justice could form the basis for impeachment. Any such step would face a steep hurdle as it would require approval by the U.S. House of Representatives, which is controlled by Trump's fellow Republicans.

According to the Washington Post, Dan Coats, the director of national intelligence, Mike Rogers, the head of the National Security Agency, and Richard Ledgett, the former deputy director at the NSA, agreed to be interviewed by Mueller's investigators as early as this week. It cited five people briefed on the requests by Mueller's team who spoke on condition of anonymity.

The emergence of the obstruction of justice inquiry may make it harder for Trump to have Mueller removed. On Monday, a Trump friend said the president was considering dismissing Mueller though the White House later said he had no plans to do so.

Moscow has denied US intelligence agencies' conclusion that it interfered in last year's election campaign to try to tilt the vote in Trump's favour.

The White House has denied any collusion, and Trump has repeatedly complained about the probe, saying Democrats cannot accept his election win. The investigations, however, have cast a shadow over his five-month presidency.

PUTIN REITERATES DENIAL

On Thursday, Russian President Vladimir Putin said 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," he said.

Putin also echoed Trump's criticism of Comey, saying it was"very strange" for a former FBI chief to leak details of his conversations with the US president to the media through a friend of his.

The obstruction of justice investigation into Trump began days after Comey was fired on May 9, according to people familiar with the matter, the Washington Post said.

The administration initially gave differing reasons for his dismissal, including that he had lost the confidence of the FBI. Trump later contradicted his own staff, saying on May 11 he had the Russia issue in mind when he fired Comey.

Comey told the Senate Intelligence Committee on June 8 he believed Trump had directed him in February to drop an FBI probe into former national security adviser, Michael Flynn, that was part of the broader Russia investigation.

Several US congressional committees are also looking into the question of Russian election interference and possible Trump campaign collusion.

Adam Schiff, the top Democrat on the House intelligence committee, would not comment on whether Mueller was looking at possible obstruction of justice by Trump but said Congress"certainly needs to get to the bottom" of the issue.

Separately, the House committee said former Homeland Security Secretary Jeh Johnson would appear at a June 21 public hearing on the Russia probe.

Mark Corallo, a spokesman for Trump's legal team, denounced the Post report, saying on Wednesday: "The FBI leak of information regarding the president is outrageous, inexcusable and illegal."

It was not clear why he attributed the report to an FBI leak. The Post report did not name the FBI as its source.

A spokesman for Mueller's team declined to comment on Wednesday.