FBI director James Comey wrote in a memo that President Donald Trump had asked him to shut down an FBI investigation into ousted national security adviser Michael Flynn, a person familiar with the situation said.

The existence of the memo was first reported on Tuesday by The New York Times. The White House denied the report.

"While the president has repeatedly expressed his view that General Flynn is a decent man who served and protected our country, the president has never asked Mr Comey or anyone else to end any investigation, including any investigation involving General Flynn," the White House said in a statement.

Mr Trump abruptly fired Mr Comey last week, saying he did so based on his very public handling of the Hillary Clinton email probe.

But the White House has provided differing accounts of the firing. And politicians have alleged that the sudden ouster was an attempt to stifle the bureau's investigation into Trump associates' ties to Russia's alleged meddling in the 2016 presidential election.

Mr Comey's memo detailing his conversation with Mr Trump would be the clearest proof to date that the president has tried to influence that investigation. The Times said it was part of a paper trail Mr Comey created documenting what he saw as Mr Trump's efforts to improperly interfere in the ongoing probe.

The Justice Department declined to comment.

According to the Times, Mr Comey wrote in the February memo that Mr Trump told him Flynn had done nothing wrong. But Mr Comey did not say anything to Mr Trump about limiting the investigation, replying, "I agree he is a good guy."

The newspaper said Mr Comey was in the Oval Office that day with other national security officials for a terrorism threat briefing. When that ended, Mr Trump asked everyone to leave except Mr Comey, and he eventually turned the conversation to Mr Flynn.

On Tuesday, for the second night in a row, Senate Republicans and Democrats were caught off-guard as they entered the chamber for a scheduled vote.

"I don't know the facts, so I really want to wait until I find out what the facts are before commenting," Senator John Cornyn told reporters.

Asked if it would be obstructing justice for Mr Trump to have asked Mr Comey to drop the Flynn investigation, Mr Cornyn said: "You know, that's a very serious charge. I wouldn't want to answer a hypothetical question."

Senator Lindsey Graham emphatically said he is not commenting on news stories any more.

"Let's get to the bottom of what happened with the director. And the best way to get to the bottom of it, is for him to testify... I'm not going to take a memo, I want the guy to come in," Mr Graham told reporters, adding, "If he felt confident enough to write it down, he should come in and tell us about it."

Senator Dianne Feinstein said Mr Comey needs to come to Capitol Hill and testify.

Senator Mark Warner of Virginia, the top Democrat on the Senate Intelligence Committee, said he will ask Mr Comey for additional material as part of the panel's investigation. "Memos, transcripts, tapes - the list keeps getting longer," he said.

Democratic Senator Chris Murphy of Connecticut tweeted: "Just leaving Senate floor. Lots of chatter from Ds and Rs about the exact definition of 'obstruction of justice.'"

There is no sign the FBI's Russia investigation is closing. Acting FBI director Andrew McCabe told Congress last week the investigation is "highly significant" and said Mr Comey's dismissal would do nothing to impede the probe.

AP

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