Donald Trump's longtime adviser Roger Stone was sentenced to 40 months in prison as part of a case that has roiled the Justice Department and drawn the US president's ire.

Stone was convicted on charges including lying to a congressional panel investigating Russian interference in the 2016 United States presidential election.

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US District Judge Amy Berman Jackson handed down the sentence after Stone's lawyer asked that the veteran Republican operative receive no prison time.

Stone's belligerence and lies represent "a threat to our democracy", the judge said in a stern lecture during the hours-long sentencing hearing.

"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.

"There was nothing unfair, phoney or disgraceful about the investigation or the prosecution," Jackson added, citing words that Trump has used.

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 toward the court," Jackson said.

"This is intolerable to the administration of justice," she added.

'Not one isolated incident'

The initial sentencing memo by the original prosecutors in the case that called for seven to nine years in prison - later reversed by the Justice Department after Trump complained publicly - was thorough and well researched, the judge said, but added that such a sentence would be "unnecessary" for Stone.

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

Stone's lawyer, Seth Ginsberg, said Stone's career as a self-described "dirty trickster" overshadowed other aspects of a spiritual man with no prior criminal record 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.

Jackson also said she would not discount tougher sentencing guidelines that apply to witness tampering and obstruction, which were among the seven criminal counts on which Stone was convicted in November.

The judge noted that Stone was not charged with or convicted of having any role in conspiring with Russia. But the judge 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 Republican.

A jury of nine women and three men convicted Stone, 67, on November 15 on all seven counts of lying to Congress, obstruction of justice and witness tampering. The charges stemmed from special counsel Robert Mueller's investigation which detailed Russian meddling in the 2016 election to boost Trump's candidacy. Stone was one of several Trump associates charged in Mueller's inquiry.

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

An inflatable rat depicted as US President Donald Trump is seen outside federal court, next to a sign demanding a pardon for Stone [Leah Millis/Reuters]

Trump, who on Tuesday granted clemency to prominent convicted white-collar criminals including financier Michael Milken and former Illinois Governor Rod Blagojevich, has sidestepped questions about whether he will pardon Stone.

"We're going to see what happens," Trump said on Tuesday.

Trump's attacks

Trump, emboldened after his Senate acquittal in his impeachment trial, has attacked the prosecutors, jurors and judge in Stone's case. After prosecutors last week recommended that the judge sentence Stone to serve seven to nine years in prison, Trump blasted them as "corrupt" and railed against this "miscarriage of justice".

Attorney General William Barr intervened and the Justice Department withdrew the sentencing recommendation, prompting the four prosecutors to resign from the case. Congressional Democrats have accused Trump and Barr of politicising the criminal justice system and threatening the rule of law.

Trump kept up his attacks even after Barr said in an ABC News interview that Trump's comments "make it impossible for me to do my job". Barr has considered stepping down, according to a source familiar with the matter.

Stone, who has also labelled himself an "agent provocateur" and famously has the face of former President Richard Nixon tattooed on his back, was arrested in January 2019 in a pre-dawn FBI raid on his Florida home.

He repeatedly pushed the boundaries set by Jackson. He violated her orders not to talk about the case or post on social media, and the judge accused him of "middle school" behaviour. At one point, Stone posted an image of Jackson on Instagram with what looked like the crosshairs of a gun over her head, later apologising to the judge in court.

The sentencing caps a roller coaster of a case that featured references to the 1974 film The Godfather Part II, a Bernie Sanders impression and testimony from figures in Trump's political inner circle including former White House strategist Steve Bannon and former Trump campaign deputy Rick Gates.