WASHINGTON - U.S. President Donald Trump lashed out on Thursday after a report that he was under investigation for possible obstruction of justice and he dismissed as "phony" the idea his campaign colluded with any Russian effort to sway the 2016 U.S. 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."

Eight hours later, after a busy day meeting with governors and witnessing the formal investiture of his pick for the Supreme Court, Trump returned to the topic again in a pair of tweets suggesting that Hillary Clinton, his Democratic opponent in the election, should be under investigation instead of him.

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

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

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Comey, who was fired on May 9, told a Senate panel last week he believed Trump dismissed him to undermine the FBI's Russia probe. He also told the Senate Intelligence Committee in his June 8 testimony that he believed Trump had directed him to drop a related agency investigation into the president's former national security adviser, Michael Flynn.

Regardless of the outcome, the specter of Mueller examining the possibility of obstruction of justice appeared to be a new blow to Trump, whose first five months in office have been clouded by the federal and congressional probes into the Russia issue.

Although he was strongly critical of some of Comey's testimony, 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.

The special counsel is following two major lines of investigation, said one U.S. official familiar with the rough outlines of Mueller's probe who spoke on condition of anonymity.

The first is into whether anyone on Trump's 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, said the official.

The second, the official continued, is into whether, if any potential offenses were committed, 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 Rod Rosenstein and possibly 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. 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.

'IT'S NOT A WITCH HUNT'

Moscow has denied U.S. intelligence agencies' conclusion that it interfered in last year's U.S. presidential election campaign to try to tilt the vote in Trump's favor.

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

Mueller was named by Rosenstein on May 17 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.

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 although the White House said later he had no plans to do so.

Republican Senator John Thune defended Mueller's integrity and said he needs to be able to continue to do his job and get to the bottom of the issues.

"It's not a witch hunt," Thune told MSNBC in an interview on Thursday. "It is in everybody's best interests if we let him do his job."

PUTIN REITERATES DENIAL

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

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 U.S. president to the media through a friend.

The obstruction of justice investigation into Trump began days after Comey was fired, 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.

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.