US president Donald Trump speaks in the Diplomatic Room of the White House in Washington, DC. Photograph: Andrew Harnik/AP Photo

Former director of the FBI accuses the Trump administration of defamation during his testimony to the US senate intelligence committee. Video: C-Span

US president Donald Trump has said he is the victim of a “witch-hunt” following reports that special counsel Robert Mueller is investigating the president for possible obstruction of justice.

In a series of early morning tweets on Thursday, the president wrote: “You are witnessing the single greatest WITCH HUNT in American political history – led by some very bad and conflicted people!”

He described the reports about obstruction of justice as a “phony story”. “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,” he tweeted.

The Washington Post reported that Mr Mueller, the special counsel appointed to oversee the FBI investigation into Russian interference in the US elections, is investigating Mr Trump for possible obstruction of justice.

The report, which cites five unnamed officials, states that the investigation into Mr Trump began after FBI director James Comey was fired by the president on May 9th.

Mr Comey confirmed in his testimony to the Senate intelligence committee last week that Mr Trump was not personally under investigation for contacts with Russia, despite an ongoing probe into links between his campaign team and the Kremlin.

According to the Washington Post report, the widening of the investigation to include possible obstruction of justice by the president began after Mr Comey was fired and Mr Mueller was appointed to lead the year-long FBI investigation.

Mr Mueller is due to interview senior intelligence chiefs, including Mike Rogers, the head of the NSA, and director of national intelligence Dan Coats, as early as this week, the report claimed. During testimony last week, both men declined to answer questions as to whether Mr Trump had tried to intervene in the Russia investigation.

Responding to the report, a spokesman for Mr Trump’s lawyer said: “The FBI leak of information regarding the president is outrageous, inexcusable and illegal.”

Mr Comey told the Senate committee last week that he believed the reason he was fired was because of the Russia investigation, and not his handling of the Hillary Clinton email scandal, as initially asserted by the White House.

“I know I was fired because of something about the way I was conducting the Russia investigation was in some way putting pressure on him, in some way irritating him, and he decided to fire me because of that,” Mr Comey said.

Although Mr Comey testified that he felt that Mr Trump was directing him to back off an FBI investigation into former national security adviser Mike Flynn during a conversation in the Oval Office on February 14th, he declined to say if this amounted to obstruction of justice – an impeachable offence. Instead, that was a matter for special counsel Mr Mueller, he said.

Mr Mueller was appointed by deputy attorney general Rod Rosenstein to oversee the Russia investigation in the wake of Mr Comey’s controversial firing. A former FBI chief, he was appointed by Republican president George W Bush but was asked to stay on for two years by Democratic president Barack Obama.

Mr Comey said last week that he had already passed on the memos detailing his meetings with the US president to Mr Mueller.

The special counsel’s independence was questioned by some Republicans this week, including former House speaker Newt Gingrich, following reports that three of the staff he appointed previously donated to the Democratic Party.

But the White House has played down comments from Trump ally Christopher Ruddy this week which suggested that the president was considering firing Mr Mueller.