The World Health Organisation on Wednesday tightened guidelines on wearing face masks.
In more detailed advice, the WHO said that, where the epidemic was spreading, people - including children and students aged 12 or over - should always wear masks in shops, workplaces and schools that lack adequate ventilation, and when receiving visitors at home in poorly ventilated rooms.
Masks should also be worn outdoors and in well ventilated indoor spaces where physical distancing of at least one meter (3 ft) could not be maintained.
In all scenarios, masks - which protect against transmission of the virus rather than infection - needed to be accompanied by other precautions such as hand-washing, the WHO said.
In areas of COVID-19 spread, it also advised "universal" wearing of medical masks in health care facilities, including when caring for other patients.
The advice applied to visitors, outpatients and to common areas such as cafeterias and staff rooms.
Health care workers could wear N95 respirator masks if available when caring for COVID-19 patients, but their only proven protection is when they are doing aerosol-generating procedures which carry higher risks, the WHO said.
It recommended that people doing vigorous physical activity not wear masks, citing some associated risks, particularly for people with asthma.
Adequate ventilation, physical distancing and disinfection of "high-touch surfaces" in the gym must be maintained, or their temporarily closure should be considered, it added.
The last advisory for masks were issued in June, when the WHO had urged governments to ask everyone to wear fabric masks in indoor and outdoor public areas where there was a risk of transmission of the virus.
Since then, a second global wave of the epidemic has gathered pace. In all, more than 63 million people globally have caught COVID-19 and 1.475 million died of it, according to a Reuters tally.