Almost a quarter of a million children in parts of Nigeria's Borno state formerly controlled by Boko Haram are suffering from severe malnutrition, the UN children's agency says.
Tens of thousands will die if treatment does not reach them soon, Unicef warns.
In areas where Boko Haram militants had been in control, it found people without water, food or sanitation.
Last month, a charity said people fleeing Boko Haram had starved to death.
The Islamist group's seven-year rebellion has left 20,000 people dead and more than two million displaced.
Nigeria's military is involved in a large-scale offensive against the group.
Unicef says that as more areas in north-eastern Nigeria become accessible to humanitarian help, the extent of the nutrition crisis affecting children is becoming more apparent.
It said that of the 244,000 children found to be suffering from severe acute malnutrition in Borno, almost one in five would die if they were not reached with treatment.