Sacked FBI Director James Comey has revealed that President Donald Trump asked him to drop an investigation into former National Security Adviser Michael Flynn.

The revelation is contained in a written statement that Mr Comey will deliver on Thursday to a Senate hearing investigating allegations of collusion between the Trump campaign and Russian officials.

Mr Comey, who was fired by the president a month ago, describes a meeting at the White House on Valentine's Day during which Trump brought up Mr Flynn.

"He is a good guy and has been through a lot," Mr Comey reports Trump as saying. "I hope you can see your way clear to letting this go, to letting Flynn go. He is a good guy. I hope you can let this go."

Mr Flynn resigned after it was revealed he had misled the White House over contacts with the Russian ambassador to the US.

Of Trump's words, Mr Comey writes: "It was very concerning, given the FBI's role as an independent investigative agency."