US President Donald Trump leaves a news conference on the novel coronavirus, COVID-19, in the Rose Garden of the White House in Washington, DC on May 11, 2020. Photo: AFP

Joe Biden attacked US President Donald Trump as an "absolute fool" Tuesday for belittling his election rival over recently wearing a mask, an issue that has become a partisan flash point during the coronavirus pandemic.

The Democratic Party's presumptive 2020 presidential nominee had a series of choice words for the Republican incumbent, telling CNN in his first in-person interview since March that Trump has become increasingly erratic.

The interview, conducted in Biden's garden and with social distancing measures in place, followed Memorial Day ceremonies Monday in which Biden wore a mask and Trump was bare-faced.

Trump retweeted a photograph of Biden and an accompanying message that mocked the former vice president for wearing a mask.

"He's a fool, an absolute fool, to talk that way," Biden said. "Every leading doc in the world is saying you should wear a mask when you're in a crowd."

Asked whether wearing a mask projected weakness or strength, Biden chose a different description.

"It presents and projects as leadership," Biden said. "Presidents are supposed to lead, not engage in folly and be falsely masculine."

Trump has refused to appear in public wearing a mask, and many of the Americans who are bare-faced at demonstrations calling for states to immediately reopen are supporters of the president.

"This macho stuff," Biden added, has "cost people's lives."

An hour before the interview aired, Trump questioned why Biden would wear a mask outdoors, in "perfect conditions, perfect weather," while he does not wear a mask in his home with his wife.

"So I thought it was very unusual that he had one on," Trump said at the White House.

Trump has repeatedly mocked Biden as too old or gaffe-prone to wrest the White House from him in November's election. 

Biden said he did not want to engage in name-calling with a president who is all-too-willing to do so, but he did not hesitate to point out Trump's apparent anxiety over slippage in the polls.

On Tuesday Biden changed the profile image on his Twitter feed to a photograph of him wearing a black mask.

Twitter Tuesday labelled two of Trump's tweets "unsubstantiated" and accused him of making false claims, a first for the social network which has long resisted calls to censure the US president over truth-defying posts.

Trump's tweets contended without evidence that mail-in voting would lead to fraud and a "Rigged Election."

Under the tweets, Twitter posted a link which read "Get the facts about mail-in ballots" and which took users to a notice pointing out that the claims are "unsubstantiated," citing reporting by CNN, the Washington Post and other media.

AFP