Footage has emerged of Democratic presidential candidate Hillary Clinton seemingly faint as she is carried into a van by her aides.

Hillary Clinton's doctor says the US Democratic presidential nominee was diagnosed on Friday (Saturday NZT) with pneumonia, put on antibiotics and advised to rest and modify her campaign schedule.

Clinton abruptly left a 9/11 anniversary ceremony Sunday after feeling "overheated," according to her campaign

A video showed Clinton slumping and being held up by three people as she was helped into a van after the event, and her doctor said in a statement that Clinton had become overheated and dehydrated.

BRIAN SNYDER/REUTERS Hillary Clinton was all smiles as she left her daughter's house.

"I have just examined her and she is now rehydrated and recovering nicely," Dr Lisa R Bardack said in a statement.

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The physician said Clinton has had an allergy-related cough, and that during a follow-up examination on Friday, the candidate was diagnosed with pneumonia, put on antibiotics, advised to rest and modify her schedule.

MORNING REPORT/Radio New Zealand Hillary Clinton overheats during September 11 commemorations, sparking concerns from her opponents about the state of her physical fitness. Our correspondent tells us the health of each candidate is a hot topic this election.

Less than two months from Election Day, it was an unwanted visual for Clinton as she tries to project the strength and vigor needed for one of the world's most demanding jobs.

Republican rival Donald Trump has spent months questioning Clinton's health, saying she doesn't have the stamina to be president.

Clinton's departure from the event was not witnessed by the reporters who travel with her campaign and aides provided no information about why she left or her whereabouts for nearly two hours.

REUTERS Democratic presidential nominee Hillary Clinton says she is feeling "great" after she became "overheated" at a September 11 memorial ceremony in New York City and departed the event early. Jillian Kitchener reports.

Spokesman Nick Merrill eventually said Clinton had gone to her daughter's nearby apartment, but refused to say whether the former secretary of state had required medical attention.

Clinton exited the apartment on her own shortly before noon. She waved to reporters and said, "I'm feeling great. It's a beautiful day in New York."

The video of Clinton posted to social media shows her being held up by aides as a black van pulls up. She stumbles and appears to fall off the curb as she is helped to the vehicle.

BRIAN SNYDER/REUTERS US Democratic presidential candidate Hillary Clinton arrives for the ceremony at the National 9/11 Memorial in New York.

After leaving her daughter's, Clinton was driven to her home in Chappaqua, New York, and made no public appearances.

She was scheduled to fly to California on Monday morning for fundraising and it was unclear whether her schedule would change.

Trump, who attended the same event marking the 15th anniversary of the 9/11 attacks, was noticeably restrained. Asked by a reporter about Clinton's health incident, Trump said, "I don't know anything."

The incident compounds an already difficult stretch for Clinton as the presidential race enters its final stretch. Despite Trump's numerous missteps, the race remains close and many Americans view Clinton as dishonest and untrustworthy.

On Friday, Clinton told donors that "half" of rival Donald Trump's supporters are in a "basket of deplorables" - a comment that drew sharp criticism from Republicans.

Clinton later said she regretted applying that description to "half" of Trump's backers, but stuck by her assertion that the GOP nominee has given a platform to "hateful views and voices."

Now Clinton is sure to face new questions about whether she's physically fit for the presidency.

Trump and his supporters have been hinting at potential health issues for months, questioning Clinton's stamina when she takes routine days off the campaign trail and reviving questions about a concussion she sustained in December 2012 after fainting.

Her doctor attributed that episode to a stomach virus and dehydration.

Clinton's doctor reported she is fully recovered from the concussion, which led to temporary double vision and discovery of a blood clot in a vein in the space between her brain and skull.

Clinton also has experienced deep vein thrombosis, a clot usually in the leg, and takes the blood thinner Coumadin to prevent new clots.

Clinton spent about 90 minutes at the 9/11 event on Sunday, standing alongside numerous other dignitaries, including New York's Democratic senators, Chuck Schumer and Kirstin Gillibrand.

The weather was warm and humid in New York on Sunday, and there was a breeze at the crowded memorial plaza during the ceremony.

Lawmaker Joe Crowley said he spent time before the ceremony chatting with Clinton and watching her sign autographs and take pictures. He said he was standing behind her during the remembrance and "she did not seem out of the ordinary at all."

"It was stiflingly hot. I was sweating through my shirt," Crowley said. "I had to leave myself. I drank about a gallon of water."

Schumer said he also spoke with Clinton during the event and saw her leave "on her own accord."

CLINTON'S HEALTH RECORD

Clinton has released far more information about her health than Trump, who is older. But her medical report is now more than a year old, and the persistent coughing has raised doubts even before Sunday.

What the country knows from past reports is that Clinton has hypothyroidism, seasonal pollen allergies and takes blood thinners as a precaution against clots, according to a letter from Bardack released by her campaign in July 2015.

In December 2012, Clinton fainted and sustained a concussion after a stomach virus and dehydration prevented her from initially testifying before Congress, which was investigating the Benghazi, Libya, attack.

She was later found to have a clot and double vision, causing her to wear special eyeglasses.

A 2013 follow-up visit showed that Clinton had no more effects from the concussion and tested negative for all blood clotting disorders, her doctor said.

"She is in excellent physical condition and fit to serve as president of the United States," wrote Lisa Bardack, chairman of internal medicine at the Mount Kisco Medical Group.

Bardack said Clinton does not smoke or use illicit drugs and drinks alcohol occasionally.

"She eats a diet rich in lean protein, vegetables and fruits," her doctor wrote. "She exercises regularly, including yoga, swimming, walking and weight training."

At the time, she was up to date on regular colonoscopy and gynecological exams, mammograms and breast ultrasounds. Her most recent physical was conducted on March 21, 2015.

Clinton's family history includes a father who died of a stroke and a mother who died from congestive heart failure.

She has two brothers, one of whom has premature heart disease.

Trump's personal physician has said the Republican presidential nominee is in excellent health both physically and mentally. But the 70-year-old has refused to release his own health records.

Dr Harold Bornstein's report last December remains the only medical information released so far by the Trump campaign.

Bornstein told NBC News he needed just five minutes to write a glowing public assessment of Trump's health as a limousine waited to carry the letter back to Trump.

Past presidential candidates have released much more detailed information about their health than either Trump, 70, or Clinton.

For example, John McCain, the failed 2008 Republican presidential nominee, allowed reporters to see 1173 pages of medical records after concerns were raised about a cancer scare.

- AP, with McClatchy and Reuters