But the intelligence agencies did not assess whether that interference had affected the election or contributed to Trump's victory, and no evidence has emerged that votes were changed improperly.

Deanna Congileo, a spokeswoman for the Atlanta-based Carter Centre, declined to make Carter available Friday for further explanation.

Special counsel Robert Mueller's report identified two criminal schemes by Russia to interfere in the election: the hacking of Democratic email accounts and a social media campaign to spread disinformation online and sway public opinion.

But Mueller's report did not establish that Russia conspired with any Trump associates in those efforts.

Carter's comments came during a panel discussion at a retreat for donors to the Carter Centre, an organisation he founded in 1982 that has been at the forefront of election monitoring efforts around the globe for decades.

The discussion was broadcast by CSPAN.

Panel moderator and historian Jon Meacham asked Carter if he believed Trump was "an illegitimate president."

Carter replied: "Based on what I just said, which I can't retract, I'd say yes."

Carter also had harsh words for Trump about his administration's immigration policies, following reports of children and teens held by Border Patrol with inadequate food, water and sanitation.

Asked by Meacham about his views on the situation at the US border with Mexico, Carter said: "Every day we send a disgraceful signal around the world that this is what the president of the United States government stands for. And that is torture and kidnapping of little children, separation from their parents, and deprivation of those who are incarcerated."

AP