"Roger Stone is now a free man!" the White House said in a statement, days before he was to report to a federal prison to start serving his term.

Trump's act is certain to renew charges that the president intervenes in the US justice system to help friends and allies and punish critics and perceived enemies.

Stone, one of Trump's oldest confidants, was convicted last November of lying to Congress, tampering with a witness and obstructing the House investigation into whether the Trump campaign conspired with Russia to help him win the 2016 election.

The White House statement reiterated Trump's charge that Special Counsel Robert Mueller investigated an alleged crime that was never committed. It argued that Stone should therefore never have been charged in the first place.

"The simple fact is that if the Special Counsel had not been pursuing an absolutely baseless investigation, Mr Stone would not be facing time in prison," it said.

dw/acb