Democratic presidential nominee Joe Biden said Monday night that President Donald Trump is responsible for contracting coronavirus, faulting Trump for failing to wear masks and follow social distancing guidelines.

"Anybody who contracts the virus by essentially saying, 'masks don't matter, social distancing doesn't matter,' I think is responsible for what happens to them," Biden said of Trump in an NBC town hall in Miami.

The comment was Biden's most direct criticism of Trump since the President revealed early Friday morning that he had tested positive for the virus.

Holding up a mask, he said, "I view wearing this mask, not so much protecting me, but as a patriotic responsibility."

Biden said he hoped the message Americans will take from Trump's sickness is that masks and social distancing are important and can save lives. But, he said, he was concerned by Trump's tweet that Biden characterised as telling Americans, "Don't be so concerned about this, essentially."

"Don't be afraid of Covid. Don't let it dominate your life," Trump had tweeted hours before he departed from the Walter Reed medical center.

"You have about 1,000 people a day getting the Corona-virus. It's a significant concern. I hope no one walks away with the message thinking it's not a problem," Biden said. "It's a serious problem. It's an international pandemic."

He also said he "wasn't surprised" when Trump tested positive for the virus.

"For the last three months, three times a week, I'm on the telephone and on Zoom with some of the leading immunologists in the nation, and they go through everything that's happening. And so the idea that Covid does not spread in proximity when you don't have a mask on, when you're not socially dancing, when there's large groups of people, when you're inside in particular and even when you're outside, that's not surprising," Biden said.

Biden noted that more than 210,000 people in the US have died of coronavirus, and said, "The expectation is if nothing changes, we're going to have another 200,000 dead by the end of the year. That's 400,000 dead, God forbid if that happens. That's more than were killed in one year in America than four years in World War II."