Hillary Clinton left a memorial ceremony commemorating the 15th anniversary of the 9/11 attacks prematurely after suffering a "medical episode".

An unidentified law enforcement source who witnessed the incident told Fox News that the Democratic presidential nominee appeared to faint as she was making her way to her vehicle.

Clinton reportedly received assistance from her security team.

"Secretary Clinton attended the September 11th Commemoration Ceremony for just an hour and thirty minutes this morning to pay her respects and greet some of the families of the fallen," Clinton spokesperson, Nick Merrill, said in a statement.

"During the ceremony, she felt overheated so departed to go to her daughter's apartment, and is feeling much better."

One witness told Fox News that the presidential candidate's "knees buckled" and lost a shoe during the "unexpected early departure".

According to Weather.com, the temperature in New York reached a high of 84F (29C) this afternoon.

Clinton was later pictured waving outside her daughter's apartment, telling reporters she is "feeling great".

Her rival, Republican candidate, Donald Trump has repeatedly questioned Clinton's health, although he has not made any direct claims to Clinton suffering from an illness.

Last month, the Republican presidential nominee said Clinton "lacks the mental and physical stamina to take on Isis [Islamic State] and all of the many adversaries we face, not only in terrorism but in trade and every other challenge we must confront to turn our great country around."

Trump also suggested that Clinton gets tired easily and took a swipe at her "short" speeches.

"She doesn't really do that much," the New York businessman said at a speech in Milwaukee. "She'll give a speech on teleprompter and then she'll disappear. I don't know if she goes home – she goes home and goes to sleep. I think she sleeps."

In response, Clinton's campaign team accused Trump of "using shameful distractions to hide his own record" and pushing a "deranged conspiracy" about her health.

Clinton is 68 and Trump is 70.

In December 2015, Trump released a letter from his longstanding doctor which said that if he manages to beat Clinton, he "will be the healthiest individual" ever elected to the presidency.