Washington: US President Donald Trump issued a fresh denial Sunday that he asked former FBI director James Comey to halt an investigation into the conduct of his dismissed national security adviser Michael Flynn.

"I never asked Comey to stop investigating Flynn," Trump said in a pre-dawn message on Twitter.

"Just more Fake News covering another Comey lie!"

The tweet was the latest in a running commentary on the case from Trump that began Saturday, a day after Flynn pleaded guilty to lying to the FBI about his interactions with a Russian official.

The President's latest Twitter barrage also included fresh attacks on the FBI – the country's top law enforcement agency – which he described as "in tatters".

Trump fired Flynn 25 days into this administration for misrepresenting the nature of his conversations with Sergey Kislyak, then the Russian ambassador, to Vice-President Mike Pence and other administration officials.

Comey has alleged that the day after that, Trump urged him to be lenient with Flynn, producing notes that said Trump told him, "I hope you can let this go."

Trump stoked the controversy with one of his Saturday tweets in which he said part of the rationale for firing Flynn was that he had lied to the FBI.

But critics pounced Saturday on Trump, arguing that if he knew at the time of his conversation with Comey that Flynn had lied to the FBI and was under investigation, it may constitute an attempt to obstruct that investigation.

Trump lawyer John Dowd drafted the president's tweet, according to two people familiar with the message. Its authorship could reduce how significantly it communicates anything about when the President knew that Flynn had lied to the FBI, but it also raises questions about the public relations strategy of the President's chief lawyer.

Two people close to the administration described the tweet simply as sloppy and unfortunate.

Dowd declined to answer questions about how and when Trump learned of Flynn's alleged lies to the FBI, a deception that did not become public until several days after Flynn's dismissal.

As Flynn pleaded guilty Friday, he made clear that he is now cooperating with special counsel Mueller's probe into Russian meddling in last year's election and possible collusion with the Trump campaign.

Flynn's decision to cooperate with Mueller was widely seen as a sign of increasing legal peril for other White House aides and perhaps Trump himself, as the investigation has expanded beyond potential collusion with Russia to include obstruction of justice and financial crimes.

The President continued tweeting about Flynn late Saturday. In one message, he complained that it was unfair for Flynn's life to be "destroyed" for lying to the FBI, arguing that the agency pursued Democrat Hillary Clinton far less aggressively while investigating her use of a private email server as secretary of state.

Trump's commentary on the case began on Saturday morning, as he addressed reporters before leaving the White House for a fundraising trip to New York.

He said he was not worried about what Flynn might share now that he is cooperating with prosecutors, forcefully asserting that there was "absolutely no collusion" between his campaign and Russia.

In other tweets on Sunday, Trump also addressed news that Peter Strzok – the former top FBI official assigned to Mueller's investigation – was taken off that job this summer after his bosses discovered he and another member of Mueller's team had exchanged politically charged texts disparaging Trump and supporting Clinton.

Strzok was also a key player in the investigation into Clinton's use of a private email server.

Trump also retweeted a pair of posts on the subject written by Paul Sperry, a media fellow at the conservative Hoover Institution. One suggested that the current FBI director, Christopher Wray, should "clean house" due to the politicisation of the agency.

A little later, Trump promised a better FBI under his leadership.

Trump also weighed in on Sunday on a decision by ABC News to suspend investigative reporter Brian Ross for an erroneous story about Flynn, saying the network should be sued.

Ross told viewers Friday morning that Flynn was prepared to testify that Trump, as a candidate for president, told him to contact Russians.

Later in the day, Ross walked back his report, saying that the source who had provided the initial information for his story later told him that it was as President-elect, not as a candidate, that Trump asked Flynn to contact Russians.

On Saturday, ABC apologised for a "serious error."

Ross' incorrect report prompted a dramatic reaction in the financial markets.

On Sunday, Trump offered a suggestion to those who lost money. 

Washington Post