More than half a million children in the west African area wracked by Boko Haram extremists are so malnourished their lives could be in danger unless they get aid, a UN humanitarian official has said.
Toby Lanzer, the UN's humanitarian co-ordinator for the area, described a region where millions get by on one meal a day and some communities have lost all their toddlers.
Military campaigns have driven Boko Haram from much of the territory it took during a seven-year uprising that killed more than 20,000 people, displaced over two million and shocked the world with the abduction of nearly 300 schoolgirls, but as aid workers got into areas the Islamic militants had controlled, the suffering they left behind became clear.
Mr Lanzer said: "What we have seen is extraordinary. I have seen adults sapped of all energy, who are almost unable to walk. We have had villages and towns devoid of two and three and four-year-old children because they've died."
Some totally destroyed towns had been cut off from the outside world for more than three years, he said.
Nigerian President Muhammadu Buhari declared that Boko Haram was "technically defeated" in December 2015, but attacks have continued in some places.
Two female suicide bombers killed at least 30 people and wounded 67 on Friday at a north-eastern Nigerian market, in violence blamed on Boko Haram.
With Nigeria and the Lake Chad region enduring the worst humanitarian crisis in Africa, the UN is launching a more than 1 billion US dollar (£800 million) appeal and hopes a conference next month in Oslo will spur donations, Mr Lanzer said.
About 11 million people are "in desperate need" of aid, about 7.1 million are "severely food-insecure" - essentially, getting one meal a day if they can - and roughly 515,000 children are or soon will be severely, acutely malnourished, he added.
"If they don't get the help they need on time, they die."
While the Nigerian government has stepped up efforts to help, an international push is needed, Mr Lanzer said.
Mr Buhari has accused the UN and aid agencies of exaggerating the crisis to seek donations.