Mrs Clinton seemed to buckle while waiting to get into a minivan at a ceremony to mark 15 years since the 9/11 terror attacks in New York, and she needed help from her security detail.

However, in a phone interview with CNN, the Democratic candidate said she became dizzy but did not lose consciousness.

The 68-year-old said she felt immediately better after getting into her air-conditioned vehicle.

In the interview, she admitted she had earlier been diagnosed with pneumonia and was told to rest for five days. But she admitted not following "that very wise advice".

Hillary Clinton tells @CNN's @andersoncooper that she didn't faint. https://t.co/vgvI4hRnBj https://t.co/kQBnWXqjzx

— Anderson Cooper 360° (@AC360) September 13, 2016

Mrs Clinton's campaign team has said it will release more of her medical records following the incident and admitted being too slow to provide information about her condition.

The campaign initially said she left the event due to "overheating". 

"We could have done better yesterday," Clinton communications director Jennifer Palmieri tweeted yesterday, "but it is a fact that public knows more about HRC than any nominee in history."

The Guardian quotes press secretary Brian Fallon as saying: “In retrospect, we could have handled it better in providing more information. That’s on us. We regret that."

Her Republican Party rival Donald Trump has previously questioned Mrs Clinton's fitness for the presidency.

However, during a speach at a rally in Asheville, North Carolina, he was restrained on the news, saying he hoped she feels better soon.

Instead he concentrated on her remark last week that half of his supporters belonged in "a basket of deplorables".

"You cannot run for president if you have such contempt in your heart for the American voter," he said.

"You can't lead this nation if you have such a low opinion of its citizens."