NEW YORK (AFP) - Democratic White House hopeful Hillary Clinton cancelled a California campaign fund-raising trip after she fell ill at a 9/11 memorial ceremony on Sunday (Sept 11) and her doctor revealed she was diagnosed with pneumonia.

"Secretary Clinton will not be traveling to California tomorrow or Tuesday," spokesman Nick Merrill said, hours after the 68-year-old candidate abruptly left the Ground Zero memorial suffering from what her doctor called dehydration.

The incident, in which a wobbly Mrs Clinton appeared to lose her footing as she was helped into her vehicle, offered Republican Donald Trump a new opening to attack his White House rival with just 15 days before their first high-pressure presidential debate.

Mrs Clinton had been seeking to bounce back from a blunder last Friday, when she told donors that half of Mr Trump's supporters belonged in a "basket of deplorables" - so Sunday's episode was certainly ill-timed.

The former secretary of state spent 90 minutes at the ceremony in lower Manhattan, greeting some relatives of those killed in the terror strikes 15 years ago, her campaign said in a statement.

Mrs Clinton was a US senator for New York at the time of the attacks.

"During the ceremony, she felt overheated so departed to go to her daughter's apartment, and is feeling much better," the statement said.

Later, the campaign released a statement from her personal doctor Lisa Bardack, who revealed that Mrs Clinton had been diagnosed with pneumonia last Friday and was suffering from dehydration.

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

A video posted on Twitter showed Mrs Clinton appearing unsteady as she waited to get into a black van to leave the 9/11 service.

She appeared to stumble as she was helped into the vehicle, and had to be held up on either side by members of her entourage. It was a humid day in New York, with temperatures around 28 deg C.

"Many of us (had) a sigh of relief when a gust of wind would come by because it was incredibly stifling," Democratic congressman Joe Crowley, who stood near Mrs Clinton for about an hour at the ceremony, told MSNBC.

Mrs Clinton walked out of her daughter Chelsea's home a few hours later, smiling and posing for pictures with a young girl before departing for her home in Chappaqua, just northeast of Manhattan.

"I'm feeling great, it's a beautiful day in New York," she said.

Mrs Clinton has maintained a busy schedule over the past six weeks, attending three dozen fund-raising events in August.

Her schedule modification comes in the heat of a tightening race.

She was due to attend finance events in California on Monday and Tuesday.

There was no word from the campaign on whether she would go ahead with her plans to campaign in the Las Vegas area on Wednesday, or whether she might do other campaign events closer to home Monday and Tuesday.

But any loss of valuable days on the campaign trail could juxtapose poorly against an aggressively campaigning Mr Trump.

Democratic National Committee interim chair Donna Brazile wished Mrs Clinton "a speedy recovery", adding that "I look forward to seeing her back out on the campaign trail and continuing on the path to victory".

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

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

Mr Trump, 70, has said Clinton is "not strong enough to be president" and that she "lacks the mental and physical stamina to take on ISIS", referring to the Islamic State in Iraq and Syria terror group.

The root of the claims lies in 2012, when Mrs Clinton was nearing the end of her State Department tenure.

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 Mrs Clinton temporarily suffered from double vision. She later received 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.

A Monday coughing spell by Mrs Clinton prompted renewed questions about whether she is physically fit for the job.

Dr Jennifer Lawless, a professor of government at American University in Washington, said Mrs Clinton will be under pressure to show she is not laid low by illness.

"What the Clinton campaign needs to do over the course of the next several days is demonstrate her vitality and viability. She has to be at tons of events and seem very energetic," Dr Lawless told AFP.

Mr Larry Sabato, a veteran political scientist at the University of Virginia, said Mrs Clinton's team should release a full health record. "We really haven't gotten very much, essentially a letter from her doctor," he told CNN, but added that Mr Trump should be held to the same requirement.

Despite Mrs Clinton's pneumonia diagnosis, she had a whirlwind schedule last Friday that included a two-hour working meeting on national security, and a sit-down interview.

"'Powering through' illness is what women do," tweeted former Michigan governor and Clinton surrogate Jennifer Granholm.

A Washington Post-ABC News poll out Sunday shows Mrs Clinton leading Mr Trump 46 per cent to 41 per cent among likely voters.