Some 75,000 children in north-eastern Nigeria risk dying of hunger in "the few months ahead", the UN says.
UN humanitarian co-ordinator Peter Lundberg said that overall 14 million people needed humanitarian assistance in a region that was the former stronghold of Boko Haram militants.
He warned that the UN did not have enough funds to avert the crisis.
Boko Haram jihadists laid waste to the region before being pushed back by Nigerian forces in recent months.
"Currently our assessment is that 14 million people are identified as in need of humanitarian assistance" by 2017, Mr Lundberg said in Nigeria's capital Abuja on Tuesday.
He added that this figure included some 400,000 children, and that 75,000 of them "are going to die in the few months ahead of us... if we don't do something rapidly and seriously".
Tens of thousands of people have been killed and more than two million displaced since Boko Haram began its military operations in 2009 in the Borno state and other areas.
Boko Haram at a glance:
- Founded in 2002, initially focused on opposing Western-style education - Boko Haram means "Western education is forbidden" in the Hausa language
- Launched military operations in 2009
- Thousands killed, mostly in north-eastern Nigeria, and hundreds abducted, including at least 200 schoolgirls
- Joined so-called Islamic State, now calls itself IS's "West African province"
- Seized large area in north-east, where it declared caliphate
- Regional force has now retaken most territory