WASHINGTON (AFP) - President Donald Trump's former press secretary was forced to apologise to presidential hopeful Joe Biden for mocking the candidate, who has struggled with stuttering, after he touched on the matter during Thursday's (Dec 19) Democratic debate.

The bizarre episode began under the glaring hot spotlight during the Los Angeles debate, as Mr Biden - who is not widely known as a stutterer - told the audience how people with varying problems approach him for condolence or advice.

"They keep in touch with me, the little kid who says 'I-I-I-I-I-I-I-I-I-I-I can't, I-I-I can't talk. Wh-wh-what do I do?'" he told the audience.

"I have scores of these young women and men who I keep in contact with," he said.

The 77-year-old former vice-president's imitation caused a stir on social media as people questioned whether he was mocking individuals with speech impediments, had himself stuttered or was compassionately recounting a tale about a child he could sympathise with.

"I I I I I I I I I I I I I I I hhhave absolutely no idea what Biden is talking about," former White House press secretary Sarah Huckabee Sanders posted on Twitter.

She was soon forced to apologise, however, after Mr Biden shot back that he had "worked my whole life to overcome a stutter. And it's my great honour to mentor kids who have experienced the same".

Admitting that she hadn't known he had struggled with the matter, Ms Sanders deleted her tweet and replied to Mr Biden, stating: "I apologise and should have made my point respectfully."

The final prime-time debate of 2019 featured just seven of the 15 Democrats still in the contest six weeks before the first nomination ballots are cast in the state of Iowa in February.