Donald Trump's longtime ally Roger Stone was sentenced Thursday to 40 months in prison for impeding a congressional investigation, in a case that ignited a firestorm over the US president's political interference in the justice system.

Stone, a veteran Republican operative and one of Trump's oldest confidants, was convicted in November of lying to Congress, tampering with a witness and obstructing the House investigation into whether the Trump campaign conspired with Russia to cheat in the 2016 election. "The truth still exists," said US District Court Judge Amy Berman Jackson as she handed down the sentence.

"The truth, still matters. Roger Stone's insistence that it doesn't, his belligerence, his pride in his own lies are a threat to our most fundamental institutions, to the very foundation of our democracy."