WASHINGTON - US President Donald Trump asked then-FBI Director James Comey to end the agency's investigation into ties between former White House national security adviser Michael Flynn and Russia, according to The New York Times.

The new development on Tuesday followed a week of tumult at the White House after Trump fired Comey and then discussed sensitive national security information about the Islamic State with Russian Foreign Minister Sergei Lavrov.

The Comey memo, first reported by the Times raised questions about whether Trump tried to interfere with a federal investigation.

The White House quickly denied the report, saying in a statement it was "not a truthful or accurate portrayal of the conversation between the president and Mr. Comey".

Comey wrote the memo after he met in the Oval Office with Trump, the day after the Republican president fired Flynn on Feb 14 for misleading Vice-President Mike Pence about the extent of his conversations last year with Russia's ambassador, Sergei Kislyak.

"I hope you can let this go," Trump told Comey, according to a source familiar with the contents of the memo.

The Times said that during the Oval Office meeting, Trump condemned a series of government leaks to the news media and said the FBI director should consider prosecuting reporters for publishing classified information.

Coming the day after charges that Trump disclosed sensitive information to the Russians last week, the new disclosure further rattled members of Congress.

"The memo is powerful evidence of obstruction of justice and certainly merits immediate and prompt investigation by an independent special prosecutor," said Democratic US Senator Richard Blumenthal.

Experts said intent was a critical element of an obstruction of justice charge, and the president's words could be subject to interpretation and possibly put into the context of other actions, like Comey's termination.

The fact that the president apparently said he "hoped" Comey would end the Flynn investigation rather than more directly ordering it "makes for a weaker but still viable case," said Christopher Slobogin, a law professor at Vanderbilt University.

Flynn's resignation came hours after it was reported that the Justice Department had warned the White House weeks earlier that Flynn could be vulnerable to blackmail for contacts with Kislyak before Trump took office on Jan 20.

Reuters