MAIDUGURI, Nigeria — The whimpers from skeletally thin babies too weak to cry are a harbinger of worse things to come: A quarter of the children lucky enough to make it to this emergency feeding center are dying. They are the latest victims of Boko Haram’s Islamic insurgency.

No one knows how many more children are dying of starvation in refugee camps and areas too dangerous to access because of the extremists’ presence, according to Doctors Without Borders, which runs the emergency feeding center. The aid group first sounded the alarm of a humanitarian crisis of “catastrophic” proportion in northeast Nigeria as Boko Haram lost its grip on some areas and its victims began to emerge.

“These are kids that basically have been hungry all their lives, and some are so far gone that they die here in the first 24 hours,” said Jean Stowell, an American midwife in charge of the center in Maiduguri, the biggest city in this largely Muslim region. The 110-bed center has quadrupled in size in recent weeks, but each time it expands it rapidly fills.

Nearly a quarter of a million children are severely malnourished because Boko Haram has disrupted trade and farming, U.N. Assistant Secretary-General Toby Lanzer warned at a meeting in Brussels on Thursday. About 2 million people in the region have not been reached, “and we can’t assess their situation. We can estimate that it’s awful.”

One million refugees from Boko Haram are crowded into camps in Maiduguri. Outside the camps, fresh produce is cruelly bountiful. But most refugees cannot afford produce, and Nigeria’s government is investigating reports of officials stealing food aid.

Elsewhere, 1 million children are trapped in areas too dangerous to reach because of Boko Haram, the U.N. children’s agency estimates