WASHINGTON -- The US Department of Justice on Thursday dropped the criminal case against President Donald Trump's first national security adviser, Michael Flynn, who lied to the Federal Bureau of Investigation (FBI) about his contacts with Russia during Trump's presidential transition period.

The criminal charges led to Flynn's ouster by Trump in February 2017 and became part of the US investigation led by Special Counsel Robert Muller into Moscow's alleged meddling to help Trump win the presidency in 2016.

"After a considered review of all the facts and circumstances of this case, including newly discovered and disclosed information...the government has concluded that (Flynn's interview by the FBI in January 2017) was untethered to, and unjustified by, the FBI's counterintelligence investigation into Mr. Flynn," the Justice Department said in a court document.

The department said it was "not persuaded that the January 24, 2017 interview was conducted with a legitimate investigative basis and therefore does not believe Mr. Flynn's statements were material even if untrue".

The department's move will all but certain end Flynn's prosecution.

In remarks to reporters from the Oval Office on Thursday afternoon, Trump called Flynn "an even greater warrior," while bashing FBI officials as "dishonest, crooked people" and "scum".

However, the dropping of charges against Flynn immediately infuriated the Democrats.

House Speaker Nancy Pelosi said Attorney General Bill Barr's "politicization of justice knows no bounds," adding that the Justice Department that Barr leads "is dropping the case to continue to cover up for the President."

House Intelligence Committee Chairman Adam Schiff said that the dismissal "does not exonerate" Flynn, and that the decision marked "the worst politicization of the Justice Department in its history".

Having pleaded guilty to his lying to the FBI about his conversations with then Russian Ambassador to the United States Sergey Kislyak, Flynn withdrew his guilty plea earlier this year, alleging prosecutorial misconduct.