WASHINGTON (AFP) - United States President Donald Trump blasted the FBI on Saturday (Jan 12), insisting it acted "for no reason & with no proof" when it opened an investigation into whether he was acting on Russia's behalf after he fired the agency's director, Mr James Comey, in May 2017.

The New York Times reported that the FBI launched the previously undisclosed counterintelligence investigation to determine whether Mr Trump posed a national security threat, at the same time that it opened a criminal probe into possible obstruction of justice by the President.

The FBI investigation was subsequently folded into the broader probe by Special Counsel Robert Mueller into Russia's meddling in the 2016 election and possible collaboration by the Trump campaign.

No evidence has publicly emerged that Mr Trump was secretly in contact with or took direction from Russian officials, the Times said.

"Wow, just learned in the Failing New York Times that the corrupt former leaders of the FBI, almost all fired or forced to leave the agency for some very bad reasons, opened up an investigation on me, for no reason & with no proof, after I fired Lyin' James Comey, a total sleaze!" Mr Trump tweeted.

According to Mr Trump, "the FBI was in complete turmoil... because of Comey's poor leadership" and the way he handled the investigation into Mrs Hillary Clinton's use of a private server to send some government e-mails.

"My firing of James Comey was a great day for America," Mr Trump claimed, describing the former FBI director as "a Crooked Cop who is being totally protected by his best friend, Bob Mueller".

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 pledge allegiance to Mr Trump and roll back the nascent Russia investigation.

A 'WITCH HUNT'?

Mr Trump has repeatedly criticised the Mueller investigation as a "witch hunt" and views it as an attempt to besmirch the legitimacy of his presidency.

His press secretary, Ms Sarah Sanders, said in a statement on Saturday that the latest charges are "absurd", adding, "James Comey was fired because he's a disgraced partisan hack... (and) President Trump has actually been tough on Russia."

Secretary of State Mike Pompeo, who was CIA director at the time the investigation was launched, declined to comment on the New York Times report, but insisted in an interview with CBS that "the notion that President Trump is a threat to American national security is absolutely ludicrous".

Mr Mueller, meanwhile, has issued dozens of indictments and steadily chalked up convictions of some of the President's close associates - including his former national security adviser, his former personal lawyer, and his former campaign chief.

The former national security adviser, Michael Flynn, pleaded guilty to lying to investigators about his Moscow ties.

The lawyer, Michael Cohen, has been sentenced to three years in prison for multiple crimes, including felony violations of campaign finance laws that prosecutors allege were carried out under Mr Trump's direction.

And Mr Trump's former presidential 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.

Cellphone records show that Cohen was near Prague during the summer of 2016, supporting claims that he met Russian officials there during the presidential election campaign, McClatchy news service has reported.

Cohen, who will testify in Congress on Feb 7, insists that he has never been to Prague, but added in a tweet: "#Mueller knows everything!"

Manafort, meanwhile, has admitted to sharing polling data with a Russian during the 2016 presidential race, according to a court filing inadvertently made public by his lawyers. CNN reported that the intended recipients were two pro-Russia Ukrainian oligarchs.

Manafort has denied lying to investigators about his dealings with the Russian, Konstantin Kilimnik, a political consultant with alleged intelligence ties, claiming he merely forgot details during the hectic campaign.

The specifics of the Mueller allegations were not previously known publicly, having been blacked out in a heavily redacted Dec 7 filing by the prosecutor's team.

But in Manafort's response, the electronic formatting for the redaction could easily be bypassed, revealing exactly what he was accused of lying about.

Mr Mueller's grand jury investigation, meanwhile, has been extended by a judge beyond its original 18-month mandate.