Democratic US presidential candidate Hillary Clinton has been diagnosed with pneumonia and advised to rest, hours after she abruptly left the 9/11 commemoration ceremony here after feeling "overheated and dehydrated".


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".


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, replied, "Yes. Thank you very much," as she smiled, waved and got into her car.