Many more cases of polio paralysis could come to light as investigators go into areas of northeastern Nigeria where the country's first cases in the two years were confirmed on Thursday.
"There could very well be more cases," Michel Zaffran, head of polio at the World Health Organization, told reporters in a series of briefings.
He said health authorities would be carrying out detailed surveillance in the state of Borno and surrounding areas to see whether and how many other children might have been affected.
"If this virus has been circulating for several years, there's a risk many more children have been paralysed. We need carry out intensive and active surveillance to identify them."
The polio virus, which invades the nervous system and can cause irreversible paralysis within hours, spreads rapidly among children, especially in unsanitary conditions in war-torn regions, refugee camps and areas where healthcare is limited, Reuters reports.
The WHO said it is planning a large, rapid response to the disease's re-emergence in Nigeria, beginning next week with emergency vaccination campaigns in four local government areas near the two confirmed cases.