WASHINGTON (AFP) - The US Justice Department withdrew its case against former White House national security adviser Michael Flynn on Thursday (May 7), handing President Donald Trump a major political victory.

The department said in a filing that Flynn's December 2017 guilty plea for lying to the FBI in an interview over his Russia contacts was moot because the alleged lies were not significant.

It also said the FBI's original probe of him had no "legitimate investigative basis."

The decision by the Justice Department, led by close Trump ally Attorney-General William Barr, came as Flynn was fighting the court's move towards a decision on his sentence, and after public statements by Trump that Flynn was the political victim of "filthy cops."

Flynn's secret talks with the Russian ambassador to Washington in December 2016, before Trump was inaugurated, was a cornerstone of the sprawling investigation by Special Counsel Robert Mueller into Moscow's meddling in the US election.

Even though Trump fired Flynn just 22 days into his administration, the president has always claimed the investigation was a political "witch hunt" and that Flynn, a former general and head of the Defence Intelligence Agency, was a "good man."

The Justice Department filing on Thursday gave support to Trump's claim, saying there were no grounds for the original investigation.

"The government has concluded that the interview of Mr Flynn was untethered to, and unjustified by, the FBI's counterintelligence investigation into Mr Flynn - a no longer justifiably predicated investigation," it said.

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