Donald Trump’s personal lawyer is already posing a defence to a possible charge of obstructing justice as the US President continues his criticism of the Russia investigation.

Amid mounting concern over Mr Trump’s continued commentary on an ongoing criminal investigation, his lawyer John Dowd jumped in to offer a potential legal defence by claiming that a president cannot obstruct justice.

Mr Trump has posted numerous tweets in recent days attacking special counsel Robert Mueller’s investigation into Russian interference in the 2016 presidential election.

The President also seemingly admitted he knew his former national security adviser Michael Flynn lied to the FBI.

That apparent admission, along with his commentary and attacks on an ongoing criminal investigation, have caused many to accuse him of obstruction of justice, an offence that could trigger impeachment proceedings.

“The president cannot obstruct justice because he is the chief law enforcement officer under [the Constitution’s Article II] and has every right to express his view of any case,” Mr Dowd told NBC News on Tuesday morning (AEST).

Mr Dowd added that the President’s tweet on the weekend saying he knew Mr Flynn lied to the FBI “did not admit obstruction”.

“That is an ignorant and arrogant assertion,” Mr Dowd said.

Mr Flynn, a retired Army general who was a senior adviser in Mr Trump’s election campaign, pleaded guilty on Friday to lying to the FBI about his contacts with the Russian ambassador.

Mr Trump later tweeted, “I had to fire General Flynn because he lied to the Vice President and the FBI.”

“He has pled guilty to those lies. It is a shame because his actions during the transition were lawful. There was nothing to hide!” Mr Trump wrote in his Saturday tweet.

I had to fire General Flynn because he lied to the Vice President and the FBI. He has pled guilty to those lies. It is a shame because his actions during the transition were lawful. There was nothing to hide!

— Donald J. Trump (@realDonaldTrump) December 2, 2017

The tweet caused an uproar in Washington because it suggested Mr Trump knew Flynn had committed a felony.

Mr Dowd has claimed he was responsible for the Twitter post.

Former-FBI director James Comey told a Senate hearing earlier this year that Mr Trump asked him to “go easy” Mr Flynn the day after he was fired. Mr Trump has denied Mr Comey’s account.

Mr Trump on Tuesday morning said Mr Flynn is being treated unfairly over his arrest for lying to the FBI, implicitly criticising Mr Mueller’s charges against him.

“I feel badly for General Flynn,” he told reporters at the White House, before accusing his Democratic presidential rival Hillary Clinton of having lied last year.

The President did not provide evidence or detail about his accusation against Mrs Clinton, who answered questions in July 2016 about her use of a private server for government emails while she was secretary of state.

There was never any indication from the FBI that Mrs Clinton did not tell the truth.

“Hillary Clinton on the 4th of July weekend went to the FBI not under oath. It was the most incredible thing anyone’s ever seen,” Mr Trump said.

“She lied many times, nothing happened to her. Flynn lied and it’s like, they ruined his life. Very unfair.”

Moscow has denied interfering in the election and Mr Trump has denied collusion by his campaign.

On Sunday night, Mr Trump attacked the FBI in a stunning Twitter tirade in which he  described the law enforcement agency’s reputation as the “worst in history”.

“After years of Comey, with the phony and dishonest Clinton investigation (and more), running the FBI, its reputation is in Tatters — worst in History!” Mr Trump tweeted.

“But fear not, we will bring it back to greatness.”

After years of Comey, with the phony and dishonest Clinton investigation (and more), running the FBI, its reputation is in Tatters – worst in History! But fear not, we will bring it back to greatness.

— Donald J. Trump (@realDonaldTrump) December 3, 2017

The President’s attack came amid dramatic developments over the weekend that saw a veteran FBI counterintelligence agent removed from special counsel Mueller’s team because of anti-Trump text messages.

– With agencies