Court documents filed by special counsel Robert Mueller's office reveal that Manafort faces possibly the lengthiest prison term in the Russia investigation. The 69-year-old Manafort is also at serious risk of spending the rest of his life in prison if a federal judge imposes a sentence within federal guidelines.

The potential sentence stems from Manafort's conviction last year on eight felony counts related to an elaborate scheme to conceal from tax authorities the millions of dollars he earned overseas from Ukrainian political consulting. It is one of two criminal cases pending against Manafort in which he faces prison time.

Manafort, who led Trump's campaign for months during the 2016 presidential campaign, is not charged with any crimes directly related to Russian election interference, the thrust of Mueller's probe. But prosecutors have recently revealed that they remain deeply interested in his contacts during and after the campaign with an associate the FBI says has ties to Russian intelligence.

In a 27-page court filing Friday, prosecutors did not recommend a precise sentence for Manafort, but they agreed with a calculation by federal probation officials that his crimes deserve a punishment of between 19.5 and 24.5 years. They also lay out in great detail for U.S. District Judge T.S. Ellis III how they say Manafort's greed drove him to disregard American law.

Manafort has been jailed for months as he awaits his formal sentencing. His lawyers have said the incarceration has created a mental and physical strain on Manafort, who has recently used a wheelchair in court appearances and will turn 70 in April.

But Mueller's team made clear that Manafort's age should not be a consideration, nor does it eliminate the risk that he could still commit new crimes.

Prosecutors often acknowledge mitigating factors that a judge may consider on a defendant's behalf in favour of a more lenient sentence. But none exist here, prosecutors said.

Manafort also faces up to five years in prison in that separate case in Washington, where he admitted to illegally lobbying on behalf of Ukrainian political interests.

Australian Associated Press