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 Democratic debate.

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

Former US Vice President Joe Biden
Former vice president Joe Biden, pictured at the Democratic Debate on December 19, has struggled with stuttering. Photo: AFP / Frederic J. BROWN

"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?'" Biden 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 Biden shot back that he had "worked my whole life to overcome a stutter. And it's my great honor to mentor kids who have experienced the same."

Admitting that she hadn't known he had struggled with the matter, Sanders deleted her tweet and replied to Biden, stating: "I apologize 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.

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Former White House press secretary
Former White House press secretary Sarah Huckabee Sanders, pictured in March 2019, apologized after mocking Joe Biden on Twitter. Photo: AFP / SAUL LOEB