He cited research showing asymptomatic people could be spreading the coronavirus without realising it.

"When you put on that face covering, you're protecting everyone else," he said.

The mayor said it could be a scarf or a bandana or anything homemade, but it should not be a surgical mask needed by front-line medical workers.

Meanwhile, the Trump administration is formalising new guidance to recommend that most Americans wear face coverings when leaving home, in an effort to slow the spread of coronavirus.

The White House coronavirus task force is considering suggesting that non-medical masks, T-shirts or bandannas be used to cover the nose and mouth when outside the home - while medical-grade masks should be reserved for those dealing directly with the sick.

Trump, who returned a negative result after being tested again for COVID-19 using a new rapid test, indicated he would support such a recommendation.

"I would say do it, but use a scarf if you want, you know, rather than going out and getting a mask or whatever."

"It's not a bad idea, at least for a period of time," he added."

A recent study by researchers in Singapore became the latest to estimate that somewhere around 10% of new infections may be sparked by people who carry the virus but have not yet suffered symptoms.

Australian Associated Press