New York - 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 said on Monday, describing 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 2 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 in the last six months, said Toby Lanzer, the UN's humanitarian coordinator for the region."What we have seen is extraordinary," he said at a news briefing at UN headquarters. "I have seen adults sapped of all energy, who are almost unable to walk. We have had villages and towns devoid of 2- and 3- and 4-year-old children because they've died."Some totally destroyed towns had been cut off from the outside world for more than three years, Lanzer said.Nigerian President Muhammadu Buhari declared that Boko Haram was "technically defeated" in December 2015, but attacks have continued in some places.Two women suicide bombers killed 57 people and wounded 177 in December at a northeastern Nigerian market, in violence blamed on Boko Haram.Saying that Nigeria and the Lake Chad region are enduring the worst humanitarian crisis in Africa, the UN is launching a more than $1bn appeal and hopes a conference meeting next month in Oslo, Norway will spur donations, Lanzer said.About 11 million people are "in desperate need" of aid, about 7.1 million of them 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, Lanzer said."If they don't get the help they need on time, they die," he said.While the Nigerian government has stepped up efforts to help, an international effort is needed, Lanzer said.Buhari, however, has accused the UN and aid agencies of exaggerating the crisis to seek donations.