US presidential candidates Hillary Clinton and Donald Trump on Wednesday moved to show their medical fitness for the White House as Clinton released a letter from her doctor declaring her fit for the presidency and Trump taped a TV segment about his well-being.

Clinton, 68, is healthy and fit to serve as president and is recovering from non-contagious, bacterial pneumonia, her physician, Dr Lisa Bardack, said in a letter about the Democratic nominee's medical condition released by the campaign. Clinton is scheduled to return to the campaign trail on Thursday.

Trump, 70, knows he could stand to lose a few pounds but otherwise is in great health, campaign adviser Sarah Huckabee Sanders told MSNBC on Wednesday after the Republican nominee taped an episode of the "Dr Oz Show" in New York that will air on Thursday.

Bardack, in her letter about Clinton, wrote: "She is recovering well with antibiotics and rest. She continues to remain healthy and fit to serve as President of the United States."

The announcements came as Clinton spent a third day at her home in Chappaqua, New York, after falling ill on Sunday morning as she left a Sept 11 memorial in New York City. Video footage taken by a bystander showed Clinton collapsing as she attempted to get into a waiting vehicle. Her campaign said later in the day that the former secretary of state had been diagnosed with pneumonia on Sept 9.

Bardack said Clinton's pneumonia was diagnosed after a chest scan that day and she was prescribed a 10-day course of antibiotics.

Bardack said Clinton's cholesterol and blood pressure are within normal ranges. She had a normal mammogram and breast ultrasound and shows no signs of developing heart disease, which runs in her family. Clinton takes medication for an underactive thyroid, which has been stable for years, Clarinex for her allergies, a vitamin B12 supplement and the blood thinner Coumadin following a 2012 blood clot in her head.

Since then, Clinton's dosage of Coumadin has "been adjusted as needed according to regular lab testing", Bardack said.

Trump's campaign has said he will release additional medical information to the public in the coming days.

The "Dr Oz Show" on Wednesday released a short clip of its Thursday segment with Trump. In it, Trump gives Dr Mehmet Oz two letters showing the results of medical tests conducted last week. Huckabee Sanders said she did not see the summary Trump showed Oz, but "he self-admitted he could lose a few pounds".

In other campaign news, former Secretary of State Colin Powell revealed distaste for both US presidential candidates.

In an email to Reuters on Wednesday, Powell confirmed the authenticity of the thousands of hacked messages but declined further comment.

In one of the leaked emails, Powell, a Republican, called Trump "a national disgrace" and an "international pariah" who allied himself with racists questioning whether President Barack Obama was born in the United States.

"When Trump couldn't keep that up," Powell wrote in the hacked message, "he said he also wanted to see if the (birth) certificate noted that he was a Muslim. As I have said before, 'What if he was?" Muslims are born as Americans every day."

As for Clinton, Powell said in another email that the Democratic former secretary of state was "greedy" and had "unbridled ambition".

In one of the newly leaked messages, from Aug 21, Powell derided Trump's attempt to attract black voters.

In a July 24, 2014, email, Powell made clear his discomfort with Clinton, writing: "I would rather not have to vote for her, although she is a friend I respect. A 70-year person with a long track record, unbridled ambition, greedy, not transformational"

In another email, Aug. 18, 2015, he wrote, using her initials, that: "Everything HRC touches she kind of screws up with hubris."

In some of the thousands of messages posted on the website DCLeaks.com, Powell also expressed his displeasure with the Clinton campaign's efforts to equate his use of an AOL email address with her installation of a private server in her home. After it was reported that Clinton had told FBI investigators that Powell had advised her to use a personal address, he lamented that what he called her "minions" were "making a mistake trying to drag me in".