US President Donald Trump came under renewed criticism on Sunday over his latest social media posts linked to the investigation into alleged collusion between his campaign team and Russian meddling into the 2016 US election.

Republican and Democrat lawmakers warned Trump he was treading on dangerous ground by commenting on an ongoing criminal investigation, and said his latest remarks could bolster a charge of obstruction of justice.

Trump tweeted on Saturday that he had fired his former national security adviser Michael Flynn in February "because he lied to the vice president and the FBI" about his communications with Russian officials after last November's election.

The comment went beyond Trump's previous assertion that Flynn only lied to US Vice President Mike Pence.

'More fake news'

Trump's tweet was criticized as suggesting that he knew at the time that Flynn had committed a serious crime, which legal experts say could be critical if, as has been alleged, Trump later pressured his then FBI director James Comey to drop a probe into Flynn.

Read more: Why the Russia probes don't cripple Trump's foreign policy

The US president later followed up by tweeting that he never asked Comey to stop investigating Flynn's contacts with Moscow, a statement at odds with an account Comey himself has given.

"I never asked Comey to stop investigating Flynn. Just more Fake News [sic] covering another Comey lie!" he posted early Sunday, before going on to say that the FBI's reputation was now "in tatters."

Flynn resigned just 24 days into his tenure after revelations that he lied about his contact with Russia's ambassador to the US after Trump was elected president in November 2016.

Comey, meanwhile, was dismissed in May with several reasons cited, including his handling of the Hillary Clinton email scandal. But his firing came suddenly as the Russia probe was unfolding.

Obstruction of justice?

Several US lawmakers, during TV interviews on Sunday, warned Trump that his latest tweets could land him in hot water.

California Senator Dianne Feinstein, the top Democrat on the Senate Judiciary Committee, said she believes the firing of Comey came "directly because he did not agree to lift the cloud of the Russia investigation."

She told the politics show "Meet the Press" on NBC that Trump's tweets point toward "obstruction of justice."

Republican Senator Lindsey Graham warned Trump to tread cautiously, telling CBS' "Face the Nation" that the president's comments about ongoing criminal investigations come at his "own peril."

"The president knew [Flynn] had lied to the FBI, which means that when he talked to the FBI director and asked him to effectively drop this case, he knew that Flynn had committed a federal crime," Adam Schiff, senior Democrat on the House Permanent Select Committee on Intelligence, told the ABC program "This Week."

ABC News correspondent suspended

In an attempt to clear up the latest tweet controversy, Trump's attorney John Dowd told ABC News on Sunday that he had drafted the tweet about Flynn and had done so in a "sloppy" manner.

Flynn pleaded guilty of lying to investigators on Friday and agreed to cooperate with prosecutors looking into communication between Trump's inner circle and Russia before he took office.

Read more: Analysis — Despite Flynn's guilty plea, Trump presidency not over yet

This latest twist was further clouded by the suspension of ABC News senior correspondent Brian Ross on Saturday for incorrectly reporting that Flynn would testify that Trump had directed him to make contact with Russian officials during the election campaign.

The broadcaster later clarified that Trump had only directed Flynn to make contact after the election. Again taking to Twitter, Trump congratulated the network for suspending Ross.

mm/cmk (AFP, AP, dpa, Reuters)