US Democratic presidential nominee Hillary Clinton has said she was "feeling fine" a day after she was taken ill due to pneumonia, and that she was eager to jump back onto the campaign trail in her battle against Donald Trump.

"I'm feeling fine and getting better," Mrs Clinton posted on Twitter, signing with the "H" that signifies she personally wrote the message. 

"Like anyone who's ever been home sick from work, I'm just anxious to get back out there. See you on the trail soon," she added.

She is expected to return to the campaign trail as early as mid-week.

Earlier spokesman Brian Fallon said there was "no other undisclosed condition."

"The pneumonia is the extent of it," he said, a day after Mrs Clinton fell ill at a 11 September memorial event in New York and her campaign announced she had been diagnosed with pneumonia.

"I expect that by the middle to the late of the week she'll be back out there on the campaign trail," he said.

"If it was up to her, she would be out there today."

Mr Fallon also said the campaign will release fresh medical records to help "further put to rest any lingering concerns" about the former secretary of state's health.

He acknowledged that Mrs Clinton has kept up a "breakneck pace" in her campaign as she seeks to maintain an edge over her rival, Republican flagbearer Donald Trump.

"Her full intention even after getting that diagnosis on Friday was to press forward," he said.

Yesterday’s episode, in which she is seen on video wobbling and needing help getting into her vehicle, has fuelled fresh speculation and conspiracy theories on the internet, already awash with unsubstantiated rumours that Mrs Clinton may have a brain tumour, Parkinson’s disease or dementia.

A campaign statement said she "felt overheated" so she left for her daughter Chelsea's apartment.

A few hours later, Mrs Clinton walked out of her daughter's home, smiling and posing for pictures with a young girl before departing for her home in Chappaqua, just northeast of Manhattan.

Mr Fallon said that Mrs Clinton's doctor, Lisa Bardack, determined that there was no lingering effects from 2012, when a stomach virus and dehydration prompted her to faint, causing what her doctor said was a concussion.

Doctors at the time found a blood clot on the brain, but Mrs Clinton later received the all-clear.

"There was nothing here in terms of anything that was caused by what happened in 2012," Mr Fallon said.

But he admitted: "I think in retrospect we could have handled it better in terms of providing more information more quickly in the 90 minutes we were putting a priority on making sure she was okay."

US Republican party presidential candidate Donald Trump has said he hoped Mrs Clinton gets well soon and back on the campaign trail.

Mr Trump's campaign said "they want to be respectful," and that staffers have been ordered to not post anything on social media about Mrs Clinton's pneumonia diagnosis, according to broadcaster CNN. 

Any violation of the order could result in termination, CNN said, citing unnamed Trump campaign sources.

Bloomberg reported a similar story, quoting "people familiar with the planning."

Mr Trump has made an issue of Mrs Clinton's health in recent weeks and said today that he will soon release more specific information about his own health.