Democrat Hillary Clinton faced questions about her health Monday after being diagnosed with pneumonia and falling ill at a 9/11 ceremony, renewing speculation about her fitness barely eight weeks from Election Day.

The video posted on Twitter showed Clinton appearing unsteady as she waited to enter a vehicle to leave the 9/11 service.

She appeared to stumble as she was helped to 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 82 degrees Fahrenheit (28 Celsius).

A few hours later, Clinton walked out of her daughter's home, 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," Clinton said.

It was unclear if she might campaign closer to home Monday and Tuesday, or if she would campaign in the Las Vegas area as scheduled on Wednesday.

But any loss of valuable days ahead of the November 8 election could juxtapose poorly with an aggressively campaigning Trump.

  

- Silence from Trump? -

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

The real estate mogul has promoted the idea in recent weeks that Clinton has serious health problems.

The internet is awash with claims that Clinton may have a brain tumor, Parkinson's or dementia.

However Trump, known for his relentless and personal attacks, brushed off questions from reporters about Clinton's health on Sunday. And on Monday, two US media outlets reported that he will say nothing about the issue.

The Trump campaign say "they want to be respectful," and staffers have been ordered to post nothing on social media about Clinton's pneumonia, CNN reported.

Any violation of the order could result in termination, CNN said, citing unnamed Trump campaign sources. Bloomberg reported a similar story.

Trump, 70, has said in past weeks that Clinton is "not strong enough to be president" and that she "lacks the mental and physical stamina" for the job.

The root of the claims lies in 2012, when 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 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.

- 'Vitality and viability' -

American University professor Jennifer Lawless  said Clinton will be under pressure to show her vitality.

"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," Lawless told AFP.

Larry Sabato, a veteran political scientist at the University of Virginia, said 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. He added that Trump must be held to the same requirement.

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

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