US election 'alternative reality': Clinton

Hillary Clinton has opened a jar of pickles on late-night US television to prove she's fit enough to be the next US president.

Presidential nominee Hillary Clinton has opened a jar of pickles on late-night US television. (AAP)

Democrat Hillary Clinton has told a late-night television show that sometimes she feels she is in an "alternative universe" in the US presidential race against Republican Donald Trump.

"I do feel sometimes like this campaign has entered into an alternative universe," Clinton said on ABC's Jimmy Kimmel Live on Monday.

"I have to step into the alternative reality, answer questions about am I alive, how much longer will I be alive."

Clinton was for the first time responding to allegations made by Trump and some of his backers that she is suffering health problems that could be problematic in the White House should she win the November 8 election.

Both Clinton and Trump have released notes from doctors declaring them physically fit for the presidency.

Kimmel asked Clinton to open a jar of pickles as a test of her strength. She did so successfully.

"This has become one of their themes. Take my pulse while I'm talking to you - make sure I'm alive," Clinton said to Kimmel of the health rumours.

Clinton said that sometimes Trump's remarks about her - such as a recent charge that President Barack Obama and Clinton co-founded the Islamic State, which he later said was sarcasm - go beyond personal attacks and become harmful to US national security.

"There's enough evidence now that when Trump talks the way he talks it actually helps the terrorists," Clinton said. "I think it's crazy, but I think it's also harmful."

Clinton said she had begun preparing for the three presidential debates scheduled in September and October.

"I want to take it seriously," Clinton said.

"But you've got to be prepared for, like wacky stuff that comes at you and I am drawing on my experience in elementary school."

Clinton brushed off the possible release of an additional 14,900 emails that the FBI found when investigating her use of a private email server as secretary of state.

"My emails are so boring," Clinton said. "What's a few more?"

The US State Department has already reviewed some 30,068 emails from her 2009-2013 tenure and released most of them, amounting to some 55,000 pages.