More than 2.8 million people have been displaced in northeastern Nigeria and parts of Cameroon, Chad and Niger.
More than 2.8 million people have been displaced in northeastern Nigeria and parts of Cameroon, Chad and Niger, fleeing attacks by Boko Haram Islamists who have ransacked villages across the poverty-stricken region.
"If we do not act now, the human suffering will only get more extreme," Stephen O'Brien told a Security Council meeting called to discuss the crisis.
"We have to stop this — we can with will, money, urgency and coordination."
More than nine million people are in need of urgent food aid in the region — seven million of them in Nigeria.
The UN under-secretary-general for humanitarian affairs stressed that Boko Haram's violent campaign "is as much or now even more a humanitarian catastrophe as it is a security priority."
Boko Haram has coerced more than 50 children to carry out suicide bombings from January to June of this year, said O'Brien.
He expressed frustration with the lack of international attention to the plight of those affected by Boko Haram's campaign.
"I have been shouting into what feels like an empty room to highlight the dire situation in the Lake Chad Basin," he added.
O'Brien renewed calls for donor funding to support humanitarian efforts.
Only 28 percent of the UN's appeal for $279 million in funding has been pledged, and requests for funds to help Niger, Cameroon and Chad are also underfunded, he said.
The medical charity Doctors Without Borders also raised alarms, saying its teams had recently found extremely high levels of malnutrition in Nigeria's northeastern Borno state.
The group called for a major humanitarian response and said the crisis should be declared a "level 3" emergency — the most severe and large-scale according to the United Nations.
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