After calling on his Democrats to avoid "playing politics" during the coronavirus outbreak, Donald Trump lashed out at Joe Biden as he continues to find ways to raise questions about his likely general election foe's competence.

The former vice president floated a "virtual" Democratic Party nominating convention in August, should the pandemic not allow American life to return to normal -- including large gatherings -- by then. He also said he intends to wear a mask in public, unlike the president, who says it's "okay" if Americans do but it's just not for him.

That was more than enough for the ever-opportunistic and always-on-the-attack president to criticise Mr Biden.

"Joe Biden wanted the date for the Democrat National Convention moved to a later time period. Now he wants a 'Virtual' Convention, one where he doesn't have to show up. Gee, I wonder why?" Mr Trump said, appearing to allege the former vice president lacks an ability to remain coherent during a convention speech to a large arena crowd.

The president also appeared to make a scheduling announcement. Mr Biden last week said he was open to having a telephone conversation with the president about the federal government's Covid-19 response. During the economic recession of 2007-2008, then-President George W Bush hosted then-Democratic nominee Barack Obama and then-Republican nominee John McCain for a White House mini-summit as they tried to forge a consensus about what to do.

A Biden spokeswoman last week reportedly said the candidate's team would contact the White House to set up a telephone chat.

"Also, what ever happened to that phone call he told the Fake News he wanted to make to me?" Mr Trump wrote on Monday morning.

The Twitter volley came a week after Democrats pushed their Milwaukee convention to 17 August and a day after Mr Biden floated the first "virtual" nominating event.

(Mr Trump said this weekend that Republicans still intend to hold their convention to again nominate him a week later in Charlotte; both Wisconsin and North Carolina are expected to be among the states up for grabs come Election Day.)

"Well, we're going to have to do a convention. We may have to do a virtual convention. I think we should be thinking about that right now," Mr Biden told ABC News on Sunday. "The idea of holding the convention is going to be necessary.

"We may not be able to put 10, 20, 30,000 people in one place and that's very possible," he added. "Again, let's see where it is -- and what we do between now and then is going to dictate a lot of that as well. But my point is that I think you just got to follow the science."

Mr Biden is leading the president in most national polls -- but the incumbent is running competitively, typically within any given poll's margin of error -- in the six or seven battleground states that political experts say will decide the election. A Reuters/Ipsos survey released last week gave the former VP a 46 per cent to 40 per cent edge nationally.

Mr Trump is a notorious poll-watcher. He has been slipping in digs at the Democratic frontrunner since his political rallies were put on hold a few weeks back as the coronavirus began sweeping the country.

Over the weekend, he again tried to paint Mr Biden as mentally feeble.