The UN agency has already named nine countries where tainted syrups may have been on sale, after the deaths of more than 300 infants on three continents last year were linked to the drugs.

Rutendo Kuwana, the WHO team lead for incidents with substandard and falsified medicines, declined to name the six new countries the agency is working with, while investigations are still underway.

He warned that contaminated medicines could still be found for several years, because adulterated barrels of an essential ingredient may remain in warehouses. Cough syrups and the ingredient, propylene glycol, both have shelf-lives of around two years.