Donald Trump has blasted the "phony story" that he is being investigated for obstruction of justice, saying it is "the single greatest witch hunt in American political history".
The Washington Post has reported that the US president is being investigated by special counsel Robert Mueller, who is probing alleged Russian meddling in the 2016 US election and possible collusion with his campaign.
"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 on Twitter, later repeating his accusation that the probe is a "witch hunt".
Former FBI director James Comey told Congress last week he believed Mr Trump fired him in May to undermine the agency's Russia investigation.
Mr Mueller was named as a special counsel by Deputy Attorney General Rod Rosenstein, eight days after Mr Comey's dismissal to lead the Russia investigation.
The position was 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 Mr 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, Mr Rosenstein and possibly Mr Trump himself, the source told Reuters.
While he was strongly critical of some of Mr 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, Mr 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 US House of Representatives, which is controlled by Mr Trump's fellow Republicans.
The emergence of the obstruction of justice inquiry may make it harder for Mr Trump to have Mr Mueller removed. On Monday, a Trump friend said the president was considering dismissing Mr 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 Mr Trump's favour.
The White House has denied any collusion, and Mr 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.
'Very strange' situation - Putin
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", he said.
Mr Putin also echoed Mr Trump's criticism of Mr 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 administration initially gave differing reasons for his dismissal, including that he had lost the confidence of the FBI. Mr Trump later contradicted his own staff, saying on 11 May he had the Russia issue in mind when he fired Mr Comey.
Mr Comey told the Senate Intelligence Committee on 8 June he believed Mr 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 Mr Mueller was looking at possible obstruction of justice by Mr 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 21 June 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 Mr Mueller's team declined to comment on Wednesday.