WASHINGTON (BLOOMBERG) - Long-time Republican operative Roger Stone deserves to spend as long as nine years in prison for lying to the US Congress to protect President Donald Trump and other crimes, prosecutors in the United States told a judge.

Stone, who was convicted in November, is due to be sentenced on Feb 20 by US District Judge Amy Berman Jackson in Washington.

"Roger Stone obstructed Congress' investigation into Russian interference in the 2016 election, lied under oath and tampered with a witness," the US said in a filing on Monday (Feb 10).

"And when his crimes were revealed by the indictment in this case, he displayed contempt for this court and the rule of law."

If accepted by the judge, Stone's sentence would be the longest of those sent to prison as a result of Special Counsel Robert Mueller's probe of Russian meddling. Former Trump campaign chairman Paul Manafort is serving 7 1/2 years for bank fraud and other crimes, and Michael Cohen, the president's one-time personal lawyer, is doing three years behind bars.

Trump's first national security adviser, Michael Flynn, is also due to be sentenced after pleading guilty to lying to federal agents. He has asked for permission to withdraw that plea and his Feb 27 sentencing date has been cancelled by the judge.

The US case against Stone - the last person charged in Mr Mueller's 22-month probe - included evidence that Mr Trump knew about WikiLeaks' plans to release emails damaging to his rival, Mrs Hillary Clinton. Stone lied to Congress to protect the president, prosecutors said.

Stone's lawyer, Mr Bruce S. Rogow, didn't immediately return a call seeking comment on the government's recommendation.

The government said in its memorandum that Stone's sentence should be consistent with federal sentencing guidelines for his crimes, a range of 87 to 108 months.

Stone's sentence should be enhanced because he used threats of violence in his campaign to keep a possible intermediary with WikiLeaks - comedian and talk show host Randy Credico - silent about the scheme, telling Mr Credico in writing to "Prepare to die," prosecutors said.

It doesn't matter whether Stone had a "serious plan" to harm him, the US said.

"Credico testified that Stone's threats concerned him because he was worried that Stone's words, if repeated in public, might make 'other people get ideas,'" the government said.

Federal prosecutors also argued that Stone's sentence should be enhanced because after he was indicted he posted an image of the judge overseeing the case with a crosshair next to her head and violated a court order by repeatedly posting about the case on social media.

Shortly after last year's verdict, Mr Trump complained about Stone's conviction on Twitter.