Health of the candidates has become a central point in the last week
US Secretary of State John Kerry says he is "absolutely convinced" Hillary Clinton is up to being president after her health scare.
The top diplomat said he was in "no doubt" about the physical stamina of the Democratic presidential candidate, after she was forced to take three days off the campaign trail to recover from non-contagious pneumonia.
Concerns were raised over Mrs Clinton's health after she was was seen stumbling as she left a 9/11 memorial.
Her Republican rival Donald Trump has avoided directly referring to Mrs Clinton's medical problems but has made a veiled attack on her health.
He has since released a statement from his doctor, which says he is in very good condition.
Harold N Bornstein MD says that Mr Trump has been under his care since November 6th 1980.
In his statement, he says the tycoon has been in hospital only once, when he had an appendectomy at the age of 11.
He says Mr Trump weighs 236 pounds and is six feet three inches tall - which the candidate earlier admitted made him overweight for his height.
But Mr Kerry said he had full confidence in Mrs Clinton's ability to be president.
"I am absolutely convinced she is, but it's not my job to get into this race.
"I have no doubt about her physical stamina. She's strong."
Mr Kerry was speaking to Sky News following the launch of his third summit on the world's oceans and climate change.
He argued the oceans were "under siege" from a range of threats, including pollution and over-fishing.
Given their importance to human life, Mr Kerry Insisted that it posed a security challenge for all countries.
"This is a life and death issue, as much as other challenges are a life and death issues," he added.
Careful not to be drawn into the current battle for the White House, Mr Kerry would not directly comment on Mr Trump's comments, in which he had questioned global warming and indicated he could ditch the global climate deal secured in Paris.
However, the veteran politician said he believed "very, very strongly" in the science of climate change and highlighted the series of measures taken by President Barack Obama to combat pollution.
He added: "I want that to continue in the future and I hope the American people will vote in a way that reflects the urgency of these issues."