Hillary Clinton's campaign will release additional medical records this week, a campaign aide said, bowing to growing criticism about how the campaign handled news of her pneumonia diagnosis.

"We're going to be releasing additional medical information," Clinton spokesman Brian Fallon said on MSNBC.

The decision to make additional disclosures came as the campaign has come under a new round of scrutiny for a lack of transparency following Clinton's abrupt, stumbling departure from a commemoration yesterday of the 15th anniversary of the September 11, 2001, terrorist attacks.

Democrats and Republicans criticised Clinton for leaving the public and the media in the dark for much of the day, feeding rumours about Clinton's health and fuelling the perception that she is unnecessarily secretive.


Aides acknowledged that the campaign should have handled news of Clinton's dizzy spell and pneumonia diagnosis differently.

"We could have done better yesterday, but it is a fact that the public knows more about HRC than any nominee in history," wrote Clinton communications director Jennifer Palmieri on Twitter in response to the criticism, using the initials of the Democratic presidential nominee.

For more than six hours after the incident, the campaign did not reveal the diagnosis, which had come on Saturday, and left a group of reporters travelling with the campaign behind and in the dark.

Today, President Barack Obama's former campaign strategist David Axelrod gave voice to Democrats' concerns that the campaign had erred on the side of secrecy rather than transparency, further playing into a perception among voters that Clinton is untrustworthy.

"Antibiotics can take care of pneumonia," Axelrod wrote. "What's the cure for an unhealthy penchant for privacy that repeatedly creates unnecessary problems?"

After Trump's campaign manager, Kellyanne Conway, retweeted him, Axelrod wrote: "Transparency works BOTH ways, @KellyannePolls! Where r @realDonaldTrump tax returns? Health records? Secret plans?"

Fallon noted in the MSNBC interview that Clinton was "dead-set" on attending the memorial service - and kept to her full schedule on Saturday, the day of the diagnosis, but he, too, acknowledged that more information should have been forthcoming.

He explained that Clinton was "alert the whole time" after she was assisted into her motorcade van at ground zero, where she left abruptly after overheating and getting "a bit dizzy".

"In those 90 minutes, that elapsed, we could have gotten more information out more quickly," he said.

Clinton was first diagnosed with pneumonia by her doctor, Lisa Bardack, who prescribed medication and advised Clinton to rest and modify her schedule.

Clinton's schedule on Saturday showed no signs of her slowing down, and the campaign did not disclose that she had been diagnosed with a serious though treatable illness.

Clinton held a meeting with a bipartisan group of national security figures and addressed the media afterward, all by holding back a persistent cough. Later, Clinton appeared at an LGBT fundraiser in Manhattan with Barbara Streisand. It was at that event where Clinton made her controversial remark that half of Republican Donald Trump's supporters are in a "basket of deplorables".

At the memorial service, Clinton became overheated and dehydrated, according to the campaign. Video footage appeared to show her buckling as she was helped into a waiting van by her security detail.

Throughout the day yesterday, however, the campaign said little. Clinton left the event early, leaving her small group of travelling reporters behind at several points in the day. More than an hour and a half passed between Clinton's departure and the campaign's decision to inform travelling reporters of her whereabouts.

And long after the campaign finally revealed Clinton's pneumonia diagnosis, aides announced she would cancel a planned trip to California today and tomorrow.

Jim Manley, a Democratic operative who was a longtime aide to Democratic Senate Leader Harry Reid said that it was troubling, though not surprising, that as much time elapsed between Clinton's departure from the event and an explanation as to what has happened.

"As someone who has watched them operate for years, none of this comes as a surprise," Manley said. "They certainly could have been more open than they were. It fits a pattern that enemies are going to use against them."

Clinton's decision to appear at the memorial event ultimately hobbled the campaign's plans for a major shift in focus away from Trump and towards Clinton's own agenda. A planned speech on the economy in California has been temporarily scrapped, and Clinton will no longer appear at several fundraising events. She will instead remain at her home in Chappaqua, New York, to rest but will teleconference into a San Francisco fundraiser today.

Former Vermont Governor Howard Dean, who is a doctor, said he is "dismayed" by the media coverage of her illness, noting that Clinton's failure to disclose her pneumonia paled in comparison to Trump's lack of transparency.

But he urged the campaign to pare back the candidate's schedule.

"Hillary need not to be working 25-hour days, which is what she's been doing - and she can do that easily," Dean said.

Democratic presidential candidate Hillary Clinton waves as she walks from her daughter's apartment building in New York yesterday. Photo / AP
Democratic presidential candidate Hillary Clinton waves as she walks from her daughter's apartment building in New York yesterday. Photo / AP

Conway raised questions about Clinton's handling of the situation.

"Lack of transparency is an overarching theme," Conway wrote in response to a news story about the health episode.

But Palmieri noted, in response to Axelrod's criticism, that Clinton has released more medical and financial information than Trump, who has refused to release his tax returns and released a letter from a doctor that was widely criticised for lacking any concrete medical information about his health.

"In contrast to HRC, Trump has been less transparent than any nominee in modern history," Palmieri wrote.

Clinton's doctor did release a letter detailing some aspects of her health, but she has not released as much medical information as some prior presidential candidates, including Arizona Senator John McCain, who in 2008 released more than a thousand pages of medical records to be scrutinised by the media.

Perhaps anticipating this line of criticism, Trump said in a television interview today morning that he would release more, "very, very specific" medical information soon.

"Hopefully they're going to be good. I think they're going to be good," Trump said on Fox News. "I feel great."