Crops had not been tended because of the violence in the region and the situation was being aggravated by a worsening drought, according to UNICEF spokesman Damien Personnaz.
"The next 18 months will be extremely difficult at the humanitarian level," he told journalists.
"About four million people are threatened by food insecurity and one million under five year-olds are suffering or will suffer from severe malnutrition," Personnaz added.
The UNICEF official estimated that about two-thirds of the local population were "still out of reach of humanitarian networks".
"We only have access to two million people out of the six million that the region had before the conflict," he said.
Seasonal rains that normally begin to fall in some areas the middle of March had still not appeared, according to the agency.
UNICEF said it would take 18 months to plant new seeds and see them grow into productive crops.