The FBI reportedly opened an inquiry in 2017 into whether US President Donald Trump was working on behalf of Russia after he fired FBI director James Comey.

The investigation - a dual counterintelligence and criminal probe - was launched after the dismissal of Mr. Comey in May 2017, the New York Times reported, citing anonymous sources.

The counterintelligence aspect consisted of determining whether Mr. Trump was knowingly or unknowingly working for Moscow and whether he was a threat to national security, the newspaper reported.

It added the criminal portion related to Mr. Trump's firing of Mr. Comeyand whether it constituted obstruction of justice.

The FBI investigation was soon folded into special counsel Robert Mueller's inquiry into Russian interference in the 2016 vote and possible collusion between his campaign and Moscow, it said, adding that it was unclear if the counterintelligence aspect was still being pursued.

The Times said that the FBI had been suspicious of Mr. Trump's ties to Russia during the 2016 campaign.

But it held off on opening an investigation until the president sacked Mr. Comey, who refused to swear his allegiance and roll back the nascent Russia investigation, which is now being spearheaded by Mr. Mueller.

Mr. Trump's lawyer Rudy Giuliani told the Times that he had no knowledge of the inquiry but said that since it was opened a year and a half ago and they hadn't heard anything, apparently "they found nothing."

Mr. Trump, who has repeatedly and vociferously denied collusion with the Russia, has frequently criticised the Mueller investigation as a "witch hunt".

But while the president has slammed the probe as baseless, it has issued dozens of indictments and steadily chalked up convictions of some of the president's close associates - including his former personal lawyer, his ex-campaign chief and his former national security advisor.

The lawyer, Michael Cohen, has been sentenced to three years in prison for multiple crimes, including felony violations of campaign finance laws he undertook, prosecutors alleged, under Mr Trump's direction.

The former campaign chair, Paul Manafort, has been convicted in one case brought by Mr. Mueller and pleaded guilty in another, over financial crimes related to his work in Ukraine before the 2016 campaign, and for witness tampering.

And the ex-national security advisor, Michael Flynn, pleaded guilty to lying to investigators about his Moscow ties.

The investigation is set to continue to cast a cloud over the White House: a judge has given an extension to the secret grand jury empanelled in the probe, extending its original 18-month mandate.