Democratic presidential candidate Hillary Clinton on Sunday left a remembrance ceremony on the 15th anniversary of the 9/11 attacks early after feeling unwell, US media reported.

CNN and MSNBC quoted a statement from her campaign as saying she "felt overheated" and so left to go to her daughter Chelsea's apartment, where she was "feeling much better."

Fox News reported, citing a law enforcement source, that Clinton appeared to have suffered a "medical episode" as she boarded her van to leave Ground Zero.