WASHINGTON (REUTERS) - A federal judge on Thursday (Feb 20) sentenced President Donald Trump's long-time adviser Roger Stone to three years and four months in prison and said his lies to lawmakers investigating Russian interference in the 2016 US presidential election amounted to a threat to American democracy.

In a stern lecture during a two-and-a-half-hour sentencing hearing for the veteran Republican operative, US District Judge Amy Berman Jackson also delivered an implicit rebuke to Trump, who has attacked her along with the jury and prosecutors in the high-profile case.

"There was nothing unfair, phony or disgraceful about the investigation or the prosecution," Jackson said, citing words that the Republican president has used.

Stone's lawyer had asked that he get no prison time. The 67-year-old Stone, who has been a friend and adviser to Trump for decades, was convicted on Nov 15 on all seven counts of lying to Congress, obstruction of justice and witness tampering.

"He was not prosecuted - as some have complained - for standing up for the president. He was prosecuted for covering up for the president," Jackson said.

"The truth still exists. The truth still matters," Jackson added. "Roger Stone's insistence that it doesn't, his belligerence, his pride in his own lies are a threat to our fundamental institutions - to the very foundation of our democracy."

The judge also said Stone "knew exactly what he was doing" when he posted an image on social media last year that positioned a gun's cross-hairs over her head.

"The defendant engaged in threatening and intimidating conduct towards the court," Jackson said. "This is intolerable to the administration of justice."

Stone, who still has a sealed pending motion requesting a new trial, declined to speak at the hearing.

Clad in a dark grey pinstripe suit with a polka dot handkerchief in the pocket, Stone stood at a lectern as the judge announced the sentence.

After leaving, Stone told reporters, "I have nothing to say."

In a chaotic scene outside the courthouse, Stone walked through a throng of people with a slight smile on his face and climbed into a waiting vehicle.

Trump has stoked speculation he may pardon Stone. A prominent Trump ally, Republican Senator Lindsey Graham, wrote on Twitter after the sentencing that the president has "all the authority in the world" to review the case.

Democratic congressman Adam Schiff, who now heads the US House of Representatives Intelligence Committee to which Stone was convicted of lying, wrote on Twitter, "It should go without saying, but to pardon Stone when his crimes were committed to protect Trump would be a breathtaking act of corruption."

The White House had no immediate reaction.

On Tuesday, Trump granted clemency to prominent convicted white-collar criminals including financier Michael Milken and former Illinois Governor Rod Blagojevich.

Jackson's sentence fell well short of the seven to nine years that were initially recommended by the original prosecutors in the case before they were overruled by the Justice Department after Trump complained publicly. Those prosecutors quit the case. The judge said such a sentence was "unnecessary" for Stone, who has no prior criminal record.

"This was still a very substantial sentence, especially for a non-violent, first time offender of his age," said Mark Allenbaugh, a consultant who formerly worked for the US Sentencing Commission.

'A HUMAN BEING'

Defence attorney Seth Ginsberg said Stone's career as a self-described "dirty trickster" overshadowed other aspects of a spiritual man who has served as a mentor, loves animals and is devoted to his family.

"Mr Stone is, in fact, not simply that public persona, but a human being," he said.

Stone was convicted of lying to the Intelligence Committee about his attempts to contact WikiLeaks, the website that released damaging emails about Trump's 2016 Democratic election rival Hillary Clinton that US intelligence officials have concluded were stolen by Russian hackers.

The judge noted that Stone was not charged with or convicted of having any role in conspiring with Russia. But Jackson said Stone's effort to obstruct a congressional investigation into Russian election meddling "was deliberate, planned - not one isolated incident."

The investigators were not some "secret anti-Trump cabal," the judge said, but members of Congress from both parties at the time when the committee was controlled by the president's fellow Republicans.

Stone's career as a Republican operative has stretched from the Watergate scandal era of the early 1970s to Trump's campaign four years ago. Stone has labeled himself an "agent provocateur"and famously has the face of former President Richard Nixon tattooed on his back.

Wearing sunglasses and a dark fedora as he entered the courthouse accompanied by an entourage of family, friends and lawyers, Stone strode past a giant inflatable rat dressed as Trump and a sign calling for his pardon. One onlooker shouted:"Traitor!"

The charges stemmed from Special Counsel Robert Mueller's investigation that detailed Russian meddling in the 2016 election to boost Trump's candidacy. Stone was one of several Trump associates charged in Mueller's inquiry.

As the hearing got under way, Trump complained on Twitter that the Justice Department should have prosecuted the former head of the FBI, James Comey, and his former deputy, Andrew McCabe, for what the president said was lying. The Justice Department investigated but opted not to prosecute either man.

"FAIRNESS?" Trump asked on Twitter.

After prosecutors made their sentencing recommendation last week, Trump blasted them as "corrupt" and railed against this"miscarriage of justice."

US Attorney-General William Barr intervened and the Justice Department overruled the recommendation. Congressional Democrats have accused Trump and Barr of politicising the US criminal justice system and threatening the rule of law.

Jackson had sharp words for John Crabb, the prosecutor newly installed on the case after the original prosecutors quit.

"I fear you know less about the case... than just about every other person in this courtroom apart from the defense attorney who just joined this team," Jackson told him.