The 15th anniversary of 9/11 should have been an opportunity for Hillary Clinton to present herself as the tested, presidential alternative to Donald Trump.

As a senator for New York during the attacks, she had toured Ground Zero, consoled grieving families and helped first responders get access to medical care.

But plans to rekindle her image of steady leadership in crisis went awry when she was forced to leave a commemoration ceremony at Ground Zero early due to a "medical episode".

Her physician later announced that Mrs Clinton is suffering from pneumonia and has been prescribed antibiotics.

"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," Dr Lisa Bardack said in a statement released by Mrs Clinton's campaign on Sunday evening.