Donald Trump’s legal problems have intensified after it was revealed that Special Counsel Robert Mueller will investigate the US President for possible obstruction of justice.

Mr Mueller is investigating alleged Russian interference in the 2016 US presidential election and the possible collusion between Russia and the Trump campaign.

As early as this week, Mr Mueller will interview three top intelligence officials as part of the probe, The Washington Post reported.

The Post, citing five unnamed officials, reported on Wednesday that 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, had agreed to be interviewed by Mr Mueller’s investigators.

CNN suggested that Mr Mueller’s investigation is “extending beyond questions of Russian interference in the 2016 election” alone.

Mr Comey last week told Congress he believed he was fired by Mr Trump to undermine the FBI’s probe into the alleged Russian links and possible collusion.

Reuters has reported that “several legal experts” believe that Mr Comey’s testimony last week — that Mr Trump expected loyalty and that he told Mr Comey he hoped he would drop an investigation of a former top aide — could strengthen the argument for an obstruction of justice case against the President.

Mr Comey declined to say in last week’s testimony whether he thought the President sought to obstruct justice.

Mr Comey noted that it was up to special counsel Mr Mueller “to sort that out”. Mr Trump’s legal team quickly denounced the Post’s latest revelations.

“The FBI leak of information regarding the President is outrageous, inexcusable and illegal,” spokesman Mark Corallo said.

A spokesman for Mr Mueller declined to comment.

After Mr Comey’s testimony, Mr Trump said he had been vindicated because his former FBI director confirmed telling Mr Trump on three occasions that he was not under investigation.

An obstruction of justice finding could form the basis for Mr Trump’s impeachment, but such a step would face a steep hurdle because it would require approval by the US House of Representatives, which is controlled by Mr Trump’s fellow Republicans.