US President Donald Trump poses with ABC anchor George Stephanopoulos at the National Constitution Center in Philadelphia, Pennsylvania on Tuesday. Photo: AFP

US President Donald Trump said Tuesday that a coronavirus vaccine may be available within a month - an acceleration of even his own optimistic predictions - but added that the pandemic could go away by itself.

"We're very close to having a vaccine," he told a town hall question-and-answer session with voters in Pennsylvania aired on ABC News.

"We're within weeks of getting it you know - could be three weeks, four weeks," he said.

Only hours earlier, speaking to Fox News, Trump had said a vaccine could come in "four weeks, it could be eight weeks."

Democrats have expressed concern that Trump is putting political pressure on government health regulators and scientists to approve a rushed vaccine in time to help turn around his uphill bid for reelection against challenger Joe Biden on November 3.

Experts including top US government infectious diseases doctor Anthony Fauci say vaccine approval is more likely toward the end of 2020.

At the ABC town hall Trump was asked why he'd downplayed the gravity of the COVID-19 pandemic, which has now killed close to 200,000 people in the US.

Trump replied by saying: "I didn't downplay it. I actually, in many ways, I up-played it in terms of action."

But Trump himself told journalist Bob Woodward during taped interviews for the new book "Rage" - published Tuesday - that he had deliberately decided to "play it down" to avoid alarming Americans.

Returning to one of his most controversial views on the virus, that has ravaged the economy and which government scientists say will remain a danger for some time, Trump insisted "it is going to disappear."

"It would go away without the vaccine but it's going to go away a lot faster with it," he said.

Challenged about how the virus would go away by itself, he said "you'll develop like a herd mentality," apparently meaning the concept of herd immunity, when enough people have developed resistance to the disease to effectively stop transmission.

"It's going to be herd developed and that's going to happen. That will all happen but with a vaccine, I think it will go away very quickly. But I really believe we're rounding the corner," he said.

Polls show that a majority of Americans disapprove of Trump's handling of the health crisis.

AFP