Hillary Clinton leaves 9/11 memorial early amid health fears

Democratic presidential candidate Hillary Clinton appeared to stumble and almost collapse after leaving a 9/11 memorial ceremony in New York on Sunday in a hurry.

A video posted on social media showed Clinton losing her footing as she waited to get into a black van to leave the emotional 9/11 service.

The Clinton campaign has since released a fresh statement saying she was suffering from pneumonia and had become overheated and dehydrated.

"Secretary Clinton has been experiencing a cough related to allergies. On Friday, during follow-up evaluation of her prolonged cough, she was diagnosed with pneumonia," Clinton's personal doctor Lisa Bardack said.

"She was put on antibiotics, and advised to rest and modify her schedule. While at this morning's event, she became overheated and dehydrated. I have just examined her and she is now re-hydrated and recovering nicely."

The former secretary of state later emerged from her daughter's nearby apartment, saying she was "feeling great", and her doctor said Clinton was "recovering nicely."

Clinton has cancelled a California fundraising trip.

"Secretary Clinton will not be traveling to California tomorrow or Tuesday," spokesman Nick Merrill said on Sunday.

The incident at Ground Zero has sparked new questions about her health - and offered Donald Trump a new opening to attack his White House rival.

Trump - who also attended the 9/11 ceremony - was uncharacteristically silent on Twitter about Clinton's illness, as both candidates took a break from formal campaigning to mark the sombre day of remembrance.

But the businessman, his spokespeople and surrogates have promoted the idea in recent weeks that Clinton has serious health problems.

Trump, 70, has said Clinton is "not strong enough to be president" and that she "lacks the mental and physical stamina to take on [the Islamic State group]".

The root of the claims lies in 2012, when Clinton was nearing the end of her stint as secretary of state. A stomach virus and dehydration prompted her to faint, causing what her doctor said was a concussion.

They said they found a blood clot on the brain and Clinton temporarily suffered from double vision. She was later given the all-clear.

The former first lady has dismissed "conspiracy theories" about her health and pointed to a letter from her doctor declaring her fit to serve as president.