US President Donald Trump issued a decision on Friday to reduce the sentence imposed on his advisor and old friend Roger Stone to avoid prison after being convicted of lying under oath before congressmen during an investigation into allegations of Russia's interference in the 2016 presidential election.

Trump's decision to reduce the punishment for Stone, 67, just days before he was imprisoned, is the most prominent intervention of the Republican president in a criminal case, and the latest incident in which he uses amnesty to help an ally.

Democrats denounced the measure, describing it as an attack on the rule of law, while the White House said in a statement that Stone had suffered greatly, and that "he had been treated very unfairly like many others in this case."

Stone, a veteran Republican politician, has been friends with Trump for decades. He was to surrender to a federal prison in Georgia state next Tuesday, to begin serving a three-year and four-month prison sentence.

Trump, who is seeking re-election as President of the United States in November, has chosen to reduce Stone's sentence, a measure that does not eliminate criminal conviction but gives full amnesty.

Stone was sentenced last February to 40 months in prison as part of an investigation into Russia's interference in the 2016 US presidential election.

Stone was convicted of seven charges, including obstructing justice, giving false testimony and tampering with witnesses, as part of a case that focused on his coordination with WikiLeaks in 2016 to publish pirated documents insulting to Hillary Clinton, Trump's rival in the then presidential election.

Stone, who was on the Trump campaign team in 2016, was arrested in January 2019 on charges brought against him by the then Special Prosecutor Robert Mueller, who was investigating Russian interference in the elections.