US Democratic presidential nominee Hillary Clinton has had health episodes in the past similar to Monday's near collapse, but has worked through them, her husband, former President Bill Clinton, says.

Hillary Clinton's campaign acknowledged today that it may have been too slow disclosing her pneumonia diagnosis after she nearly fainted at an event over the weekend, and promised to release additional medical details in the coming days.

"Rarely, on more than one occasion, over the last many, many years, the same sort of thing's happened to her when she got severely dehydrated, and she's worked like a demon, as you know, as Secretary of State, as a senator, and in the year since," Bill Clinton said in an interview with CBS.

Speaking to CNN, Hillary Clinton said she was dizzy but didn't lose consciousness when she stumbled on the way out of the 9/11 memorial on Sunday.


Clinton told CNN's Anderson Cooper 360 that she felt dizzy and "did lose my balance," while waiting for the motorcade.

But she felt immediately better after getting into her air-conditioned vehicle.

She said she didn't immediately disclose that she's been diagnosed with pneumonia because she "just didn't think it was going to be that big a deal'."

Clinton said her doctor told her to rest for five days, but she "didn't follow that very wise advice".

She added that she'll be back on the campaign trail "in the next couple of days".

The health scare revived concerns about a tendency towards secrecy that has dogged the former Secretary of State's run for the White House, and underscored perennial worries about the medical fitness of candidates for one of the world's most demanding jobs.

Both Clinton, 68, and Republican rival Donald Trump, 70, said they intended to release more of their medical details in the coming days, as their campaigns gear up for the November 8 election.

- Reuters, AAP