Donald Trump has lashed the Federal Bureau of Investigation (FBI), labelling it the "worst" it has ever been and saying its "reputation is in Tatters".

The President took aim at the agency after allegations surfaced that he attempted to pressure former FBI director James Comey to stop investigating ex-national security advisor Michael Flynn over conversations he had with Moscow's ambassador to the US.

Mr Trump said the accusation was nothing but "fake news covering another Comey lie".

However, with retired General Flynn pleading guilty to making false statements to the FBI about his discussions with Sergey Kislyak as part of special counsel Robert Mueller's probe, Mr Trump has contradicted his original reason for firing him.

At the time, the President said Mr Flynn had been removed for not being straight with vice-president Mike Pence, but his recent tweet shows he was fully aware that he had been lying to the FBI too.

“I had to fire General Flynn because he lied to the Vice President and the FBI,” he wrote on December 2.

FBI agent sidelined for anti-Trump position

Meanwhile, news outlets reported over the weekend that the deputy head of FBI counter-intelligence Peter Strzok had been stood down from the investigation into Russia's meddling in the 2016 election by Robert Mueller for allegedly sending politically-charged text messages to a colleague that denigrated Mr Trump.

Mr Strzok was also an influential figure in the probe into Hillary Clinton’s use of a private email server, casting doubt over the balance of the investigation.

In response, the President tweeted: “Tainted (no, very dishonest?) FBI “agent’s role in Clinton probe under review.” Led Clinton Email probe. @foxandfriends Clinton money going to wife of another FBI agent in charge.”