The president's tweets early on Thursday followed a Washington Post report on Wednesday citing unidentified officials that special counsel Robert Mueller was investigating Trump for possible obstruction of justice.

"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. An hour later he claimed: "You are witnessing the single greatest WITCH HUNT in American political history - led by some very bad and conflicted people!"

Robert Mueller, a former FBI director, is leading an investigation into alleged Russian meddling in the 2016 US presidential election and possible collusion with the Trump campaign. After Mueller was appointed last month, Trump tweeted: "This is the single greatest witch hunt of a politician in American history."

'Unavoidable' investigation after Comey

Reuters cited a source which confirmed the Post report, adding that an examination of possible obstruction of justice charges was "unavoidable" after former FBI Director James Comey told Congress he believed he was fired by Trump to undermine the agency's Russia investigation. Obstruction of justice is a potentially impeachable offense.

Those who have agreed to be interviewed, according to the Post, include Dan Coats, director of national intelligence; Mike Rogers, chief of the National Security Agency; and Richard Ledgett, who recently left his post as deputy to Rogers.

The Post referred to anonymous sources who had been briefed on requests made by Mueller's investigators. It was not known whether the FBI was the source of the information in the Post's report.

jm/msh (Reuters, AP)