Former FBI director James Comey is now convinced Donald Trump was trying to influence his investigation into ties between the President and Russia, a source close to him has said.

However, they told CNN it would be difficult to prove the President had committed this crime, because “you have to have intent in order to obstruct justice in the criminal sense"

In the US, obstruction of justice is a federal offence in which someone "corruptly attempts to influence, obstruct or impede the due and proper administration of the law”, they told the US broadcaster.

Mr Comey initially believed that he could instruct the new President in what was appropriate during their communications, but later became convinced the President was intending to try and affect the direction of the investigation, the source told the US broadcaster.

The former FBI director has confirmed that he will testify before the Senate Intelligence Committee on the investigation into links between Mr Trump's administration and Russia.

Mr Trump told Russian officials during an Oval Office meeting that firing Mr Comey eased pressure caused by an investigation into the Kremlin's influence in the 2016 campaign, The New York Times reported.

"I just fired the head of the FBI He was crazy, a real nut job," Mr Trump said, according to a document detailing the Russian visit. "I faced great pressure because of Russia. That's taken off."

The President also reiterated that he was not personally under investigation, although he previously admitted that he was thinking "of the Russia thing" when he dismissed Mr Comey.

Vladimir Putin responds to James Comey firing: 'We had nothing to do with it'

"I just fired the head of the FBI He was crazy, a real nut job," Mr Trump said, according to a document detailing the Russian visit. "I faced great pressure because of Russia. That's taken off."

The White House did not dispute the account portrayed, and instead said that the FBI investigation had put unnecessary pressure on the President's ability to conduct diplomacy with Russia.

But it lead to criticism from Mr Trump's political rivals, including Democratic Leader Nancy Pelosi, who said earlier this week: “If these reports are true, the President’s brazen attempt to shut down the FBI’s investigation of Michael Flynn is an assault on the rule of law that is fundamental to our democracy.