Hillary Clinton's health moved to center stage in the US presidential race on Sunday after she fell ill at a high profile 9/11 ceremony, and her doctor said the Democratic nominee had pneumonia.

Clinton left the 9/11 commemoration after 90 minutes because her campaign said she felt "overheated." A video posted online appeared to show the 68-year-old wobbling on her feet as she was helped into a van to be taken to her daughter's New York apartment.

Clinton's doctor, Lisa Bardack, said in a statement that the former secretary of state had been diagnosed with pneumonia on Friday after experiencing coughing related to allergies.

"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," 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."

Clinton emerged from her daughter's apartment after several hours saying she was "feeling great." She then went to her home in Chappaqua, 30 miles (50 km) north of New York City.

Still, Clinton's camp later said they were canceling a campaign trip to California because of the diagnosis. She was originally scheduled to leave for the state on Monday for a series of fundraisers.

The incident comes after earlier this month Clinton had a coughing fit at a campaign rally. That event added further fuel to accusation from Donald Trump and some of his backers that Clinton has undisclosed health problems and lacked the "stamina" to be president.

Those allegations have centered largely around a concussion Clinton sustained in December 2012 after fainting. The fall led to temporary double vision and discovery of a blood clot in between her brain and skull.

Her doctors have said she fully recovered. They have also said she has deep vein thrombosis and takes a blood thinner to prevent new clots.

Neither presidential candidate has released their health records.

Trump, 70, only released a letter from his doctor that said he would be "the healthiest individual ever elected to the presidency."

(DW)