Most of the children are in areas that had been inaccessible before a multinational force liberated them from Boko Haram extremists earlier this year.
A third of children in poor nations fail to meet mental development milestones, researchers say.
The seven-year Boko Haram insurgency has killed more than 20,000 people in the region.
The UNICEF statement Tuesday says the agency has received less than half of the $55.5 million it appealed for earlier this year, and now the needs are even greater.
More about:
-1780923278.jpg&h=190&w=280&zc=1&q=100)
