White House hopeful Hillary Clinton suggested on Monday that rival Donald Trump could be an unpredictable adversary in upcoming presidential debates, saying she is girding for "wacky stuff".

With the first debate scheduled to take place a month from now near New York, Clinton joked about what to expect from her unorthodox opponent.

"You've got to be prepared for wacky stuff. I'm planning on drawing off my experiences from elementary school," she said on late-night talk show Jimmy Kimmel Live.

Trump has suggested he might not attend all of the three scheduled debates.

But he is trailing badly in the polls and the highly watched events offer a much-needed chance to break through with undecided voters.

For a first-time debater, this caldron of presidential political theater could present a daunting challenge.

Trump has thrown a series of barbs at Clinton, describing her as "crooked" and making opaque suggestions lately that she has unspecified health problems.

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

Host Kimmel asked Clinton to open a jar of pickles to show she was in good health and also took her pulse as she brushed off a scandal over emails sent from a private server when she was secretary of state.

After Clinton told Kimmel that she used FaceTime video phone calls to visit with her grandchildren, he teasingly demanded whether she had ever considered using FaceTime instead of email.

Clinton also hit back at Trump's highly contested assertion that she and President Barack Obama created the Islamic State group.

"Who would have guessed this audience would be so enthusiastic about the co-founder of IS?" Kimmel asked, after a rousing welcome for Clinton from the crowd.

Clinton responded that she thought "when Trump talks the way he talks, it actually helps the terrorists".

"I do feel sometimes like this campaign has entered into an alternative universe. I have to sort of step into the alternative reality and, you know, answer questions about am I alive?" she said, referring to the health rumors.