“I don’t think I’m going to be doing it,” said Trump. “Somehow sitting in the Oval Office behind that beautiful Resolute desk, the great Resolute desk, I think wearing a face mask as I greet presidents, dictators, kings, queens – I don't know, I don't see it for myself.”

The move followed a growing movement across the country at the local level to encourage people to wear masks.

Starting on Saturday, San Diego county in southern California will become the first municipality in the state to require members of the public to wear a face covering when their jobs bring them into close proximity to others.

California this week became the first state to officially acknowledge that among the general public wearing a face covering could help prevent the spread of the coronavirus, releasing new public health guidelines to that effect.

Already mayors in Los Angeles and the San Francisco Bay Area had begun encouraging their citizens to wear face coverings when conducting essential business in public.

According to the new order in San Diego, all employees who may have contact with the public in any grocery store, pharmacy, convenience store or gas station must wear a cloth face covering.

The World Health Organisation on Friday said it backs government initiatives that require or encourage public wearing of masks, marking a major shift from the previous advice against almost all public uses of masks amid the Covid-19 pandemic.

The WHO added that surgical masks should be reserved for medical professionals, while the public should use mainly cloth or home-made masks.

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Until now, the US government and major health organisations had been staunchly opposed to widespread mask wearing, fearing that the public would horde medical-grade masks needed by beleaguered health care workers, and that masks could give the public a false sense of invulnerability to the virus, leading them to flout strict social distancing restrictions.

The more sceptical take on masks in the West is sharply at odds with countries in Asia, where mask-wearing is ubiquitous. The Czech Republic and Slovakia have also made mask-wearing mandatory.

In Texas, Laredo – a town of about 260,000 on the Mexican border – announced it would fine people over the age of five up to US$1,000 if seen in public without a face covering of some kind.

All of the new guidance around face coverings has been careful to stress that nobody who is not a front line responder or medical personnel should be wearing medical-grade masks – either surgical masks or N95 masks – as those are still desperately needed by health care workers.

Instead, the public should be wearing cloth masks, home-made masks, or even improvised face coverings like bandanas and scarves.

Announcing the new guidelines for California on Thursday, Governor Gavin Newsom stressed that masks “are not a substitute for physical distancing”.

“They are not a substitute for the stay at home order,” he said. “They are not a call for folks to buy N95 masks and surgical masks, pulling them away from … our first responders.”

Additional reporting by Stuart Lau and Owen Churchill