A member of US presidential candidate Hillary Clinton's campaign has admitted that they had mishandled the news of her pneumonia diagnosis over the weekend.

Clinton was diagnosed with the illness on Friday, but the information was not disclosed by her team until she was forced to leave a 9/11 commemoration on Sunday, and filmed stumbling into her vehicle afterwards.

Communications director Jennifer Palmieri was responding to questions from Democrats surrounding the transparency of the campaign.

"We could have done better yesterday," Palmieri said via Twitter in a rare acknowledgement of shortcomings from the campaign.

Thanks to everyone who’s reached out with well wishes! I’m feeling fine and getting better. -H — Hillary Clinton (@HillaryClinton) September 12, 2016

Press secretary Brian Fallon also admitted that the incident could have been handled better.

"In retrospect, we could have handled it better in providing more information. That’s on us. We regret that," said Fallon.

Clinton's campaign has promised to release detailed medical records to the public this week, as she comes under pressure from Republican rival Donald Trump to be more transparent about her health.

Both the Democrat and Republican candidates are amongst the oldest to have ever run for the White House. If Trump was elected he would become the oldest to begin a presidential term, at 70 years old. Clinton would be the second-oldest after Ronald Reagan.