The US government has begun advising people to wear masks if they go outside, pointing to research that shows the coronavirus could be spread just by breathing.
The advice came as America logged another huge rise in its death toll -- almost 1,500 in one day -- and as new infections continued unabated.
It also came as China -- the origin of the outbreak -- held a national day of mourning for its citizens killed by the disease.
Since COVID-19 emerged late last year, around 1.1 million people have fallen ill. Almost 60,000 people have died.
Europe accounts for the lion's share of the dead, with Italy and Spain bearing the brunt.
But the situation is rapidly deteriorating in the United States, and President Donald Trump's administration on Friday suggested widespread use of simple masks or scarves might help stem the rocketing infection rate.
"It's going to be really a voluntary thing," he said. "You don't have to do it and I'm choosing not to do it, but some people may want to do it and that's okay."
"The worst is yet to come," UN Secretary-General Antonio Guterres said, referring to countries such as Syria, Libya and Yemen. "The COVID-19 storm is now coming to all these theatres of conflict."