The WHO said on Monday that facemasks could be justified in areas where hand-washing and physical distancing were difficult, as it teamed up with Lady Gaga to launch a giant coronavirus awareness concert.
World Health Organisation chief Tedros Adhanom Ghebreyesus said masks alone though were no "silver bullet" to defeating the Covid-19 pandemic, which has killed more than 70,000 people.
"Countries could consider using masks in communities where other measures such as cleaning hands and physical distancing are harder to achieve because of lack of water or cramped living conditions," Tedros told a virtual briefing in Geneva.
"Masks should only ever be used as part of a comprehensive package of interventions There is no black or white answer, and no silver bullet. Masks alone cannot stop the Covid-19 pandemic."
However, he stressed that the mass use of medical masks could exacerbate the shortage of protective equipment for healthcare workers.
Meanwhile he teamed up with US singer Lady Gaga to announce a concert on April 18 entitled "One World: Together at Home".
It is billed as a "global broadcast and digital special to support frontline healthcare workers" and the WHO.
The gig will feature stars including Andrea Bocelli, Chris Martin, David Beckham, Elton John, John Legend, Keith Urban, Lang Lang, Paul McCartney, Priyanka Chopra Jonas, Shah Rukh Khan and Stevie Wonder.
Tedros also lashed out at "racist" suggestions from some scientists that Africa could be used as a testing ground for a vaccine, which is thought to be 12 to 18 months away.
"Africa cannot and will not be a testing ground for any vaccine," he said, adding that the suggestion was a hangover from the "colonial mentality".