Hillary Clinton diagnosed with pneumonia: Doctor

The former secretary of state has been experiencing a cough related to allergies, her doctor Lisa Bardack said in a statement.

During a follow-up evaluation of her prolonged cough on Friday, "she was diagnosed with pneumonia. She was put on antibiotics, and advised to rest and modify her schedule," Bardack said.

Clinton, 68, became "overheated and dehydrated" at the ceremony marking the 15th anniversary of the 9/11 attacks at the Ground zero memorial in lower Manhattan early yesterday, the doctor said, adding that she examined Clinton and "she is now rehydrated and recovering nicely".

In December 2013, the former first lady had to be rushed to New York's Presbyterian Hospital after a medical scare following a fainting spell and concussion.

Concerns over Clinton's health grew after she abruptly left the 9/11 memorial in downtown Manhattan where she had gone to pay respects to the victims of the terror attacks.

Her campaign said she had felt "overheated" and was taken to her daughter Chelsea's apartment in the posh Flatiron building in the city.

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

However, a video posted on Twitter by a person who was at the memorial showed Clinton, surrounded by her staff and Secret Service agents, stumble and her legs buckle as she tried to get into her car. Two Secret Service agents then held on to her arms and helped her get into the car.

It shows she could not get into the vehicle on her own and was hoisted into the car by the Secret Service agents.

Later Clinton, coming out of her daughter's apartment, waved and smiled to the people waiting outside. Asked by reporters waiting outside if she's feeling better, Clinton wearing trousers and coat, replied, "Yes. Thank you very much," as she smiled, waved and got into her car.

Merrill said the former secretary of state attended the morning 9/11 ceremony for 90 minutes "to pay her respects and greet some of the families of the fallen".

Clinton and her Republican rival Donald Trump, 70, arrived separately at the 9/11 memorial in downtown Manhattan for the 15th anniversary commemoration of the 2001 terror attacks early yesterday morning.

Several posts began appearing on Twitter that said Clinton left the ceremony abruptly as the names of the victims of the 9/11 attacks were being read out. The pool of reporters that travels with Clinton were not allowed to follow her and were prevented from leaving the media area for a period of time.

The Clinton campaign has been refuting allegations that her health is failing and has criticised Trump for "parroting lies" based on fabricated documents as he had said at an election rally that Clinton "lacks the mental and physical stamina to take on ISIS".

Earlier this month, Clinton's opponents latched on to a coughing fit she had on a campaign trail to undermine her ability to lead the country.

Clinton was afflicted by an uncontrollable cough for several minutes which forced her to stop speaking during a rally in Cleveland. She had to take cough drops and drank water while the audience waited for her to recover.

"While it is dismaying to see the Republican nominee for president push deranged conspiracy theories in a foreign policy speech, its no longer surprising," Jennifer Palmieri of the Clinton campaign had said last month.