Special prosecutor Robert Mueller has recommended that US President Donald Trump's former national security adviser Michael Flynn face no jail time due to his "substantial" co-operation with the investigation.

Mr Mueller is leading the inquiry that is investigating alleged Russia interference in the 2016 presidential election.

The former FBI director said in a court filing that Mr Flynn, who admitted last year to lying about his contacts with Russians following Mr Trump's November 2016 election victory, had helped in his and other unspecified federal criminal investigations, including being interviewed 19 times.

Mr Mueller also told the Washington Federal court that despite his "serious" offence, the retired three-star general and former Pentagon intelligence chief had a strong record of military and public service.

The surprise recommendation came ahead of Mr Flynn's upcoming sentencing, which has been postponed four times over the past year.

Those postponements indicated that, once hostile to the investigation that threatens Mr Trump and his inner circle, he had possibly become a valuable witness.

"Given the defendant's substantial assistance and other considerations set forth below, a sentence at the low end of the guideline range -- including a sentence that does not impose a term of incarceration -- is appropriate and warranted," Mr Mueller said in a memorandum to the court.

Mr Flynn's was the first guilty plea secured by the Mueller investigation into alleged collusion between Mr Trump's election campaign and Russia.

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His position as a top-level insider in the campaign, accompanying Mr Trump to key events, making a keynote speech at the July 2016 Republican convention, and then taking hold of the White House national security apparatus in Mr Trump's first weeks in office, made him a potentially extremely valuable witness.

But many thought his own problems had likely compromised that value.

In an interview with investigators on 24 January 2017, four days after Mr Trump's inauguration, Mr Flynn lied about conversations he had the previous December with Russia's ambassador to the US, Sergei Kislyak.

In those conversations, apparently recorded by US intelligence, Mr Flynn appeared to be trying to undermine the policy of then-President Barack Obama by making separate political deals with Moscow.

Mr Obama at the time was planning sanctions on Russia for its interference in the 2016 election. Mr Flynn urged the Russians not to retaliate, suggesting Trump would reverse the sanctions.

Within weeks, Mr Flynn was forced to resign after it was alleged that he also had lied to top White House officials about his Kislyak talks.

Then in March 2018, in a second interview with the FBI, Mr Flynn lied about the fact that before and after the election, he had a $530,000 lobbying contract on behalf of Turkey that he had not reported.

"At the time, the defendant was a national security advisor and surrogate for the Trump campaign who opined publicly on foreign policy and national security issues," the sentencing memorandum noted.

Despite the light sentence recommendation, the memorandum gave no hint as to what Mr Flynn had told the Mueller team about the operations of the Trump campaign and its Russia ties.

Mr Mueller has identified numerous contacts, including efforts by Trump aides during 2016 to set up a meeting between him and Russian President Vladimir Putin.