WASHINGTON • The special prosecutor in charge of the Russia election meddling probe has recommended that US President Donald Trump's former national security adviser. Michael Flynn, face no jail time due to his "substantial" cooperation with the investigation.

Mr Robert Mueller said in a court filing that 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 Flynn's upcoming sentencing, which had 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.

Flynn's was the first guilty plea secured by the Mueller investigation into alleged collusion between Mr Trump's election campaign and Russia. His position as a top-level insider in the campaign, 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 Jan 24 last year, four days after Mr Trump's inauguration, Flynn lied about conversations he had the previous December with Russia's ambassador to the US, Mr Sergei Kislyak.

In those conversations, apparently recorded by US intelligence, 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. Flynn urged the Russians not to retaliate, suggesting Mr Trump would reverse the sanctions.

Within weeks, 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 last year, in a second interview with the FBI, he lied about the fact that before and after the election, he had a US$530,000 (S$724,500) lobbying contract on behalf of Turkey that he had not reported.

"At the time, the defendant was a national security adviser 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 Flynn had told the Mueller team about the operations of the Trump campaign and its Russia ties.

In another surprise, Mr Trump's former personal lawyer, Michael Cohen, last week admitted that through the first half of 2016, even after Mr Trump had secured the Republican nomination for president, he had top-level contacts with Moscow over a project to build a potentially 100-storey Trump Tower in the Russian capital.

Mr Mueller is also investigating a June 2016 meeting that top campaign staff, including chairman Paul Manafort, Mr Trump's son Donald Jr, and son-in-law Jared Kushner, had with a Russian lawyer who had offered dirt on Mr Trump's election opponent, Mrs Hillary Clinton.

AGENCE FRANCE-PRESSE